It depends on what letter, if any, is next to the date. A 1928 C bill from the Cleveland* Federal Reserve District is quite valuable, with prices for a circulated bill in the $250 to $450 range as of 10/2011. Other letters are much more common; please see the Related Question for more.
(*) To explain, there's no "Federal Reserve of Ohio". It's actually the Fourth Federal Reserve District which has its offices in Cleveland, so it's generally referred to as the Cleveland Federal Reserve District.
The green seal indicates that it's a Federal Reserve Note. Please see the question "What is the value of a 1928 US 5 dollar Federal Reserve Note?" for more information.
The green seal indicates that your bill is a Federal Reserve Note. Please see "What is the value of a 1928 D US 5 dollar Federal Reserve note?" for more information.
The first $2 Federal Reserve Notes were issued in 1976. The banner across its top and the red seal indicate it's a United States Note, a form of paper money issued directly by the federal government until the late 1960s. Please see the question "What is the value of a 1928 D US 2 dollar bill?" for more information.
The green seal indicates that your bill is a Federal Reserve Note. All 1928 B $20 bills were printed as FRNs. Please see the question "What is the value of a 1928 B US 20 dollar bill?" for more information.
Please don't assume that every bill is a Federal Reserve Note. The banner across its top and the red seal indicate it's a United States Note, a form of paper money issued directly by the Federal government. US Notes were discontinued in the 1960s. There's more information at the question "What is the value of a 1928 G US 2 dollar bill?".
A denomination is needed. Please look for questions in the form "What is the value of a 1928 A US [denomination] dollar Federal Reserve Note?" for specific information.
The green seal indicates that it's a Federal Reserve Note. Please see the question "What is the value of a 1928 US 5 dollar Federal Reserve Note?" for more information.
The green seal indicates that your bill is a Federal Reserve Note. Please see "What is the value of a 1928 D US 5 dollar Federal Reserve note?" for more information.
All 1928 $20 Federal Reserve Notes carried the phrase "Redeemable in Gold", although they weren't gold certificates. There were also gold certificates with that denomination but they have gold seals and lack the words Federal Reserve Note. Please see the question "What is the value of a 1928 US 20 dollar Federal Reserve Note?" for more information.
"A" is the highest series letter for 1928 $50 Federal Reserve Notes. "K" is most likely the Federal Reserve District letter. The series letter, if any, on US bills is next to the date. Please see the question "What is the value of a 1928 US 50 dollar Federal Reserve Note?" for more information on values.
1928 $50 bills were issued as gold certificates and as Federal Reserve Notes but they're not the same. Please check the wording across your bill's top front as well as its seal color*; then see one of the following questions: "What is the value of a 1928 US 50 dollar Federal Reserve Note?""What is the value of a 1928 US 50 dollar gold certificate?"* Gold certificates have gold seals and FRNs have green seals.
1928-series $100 bills weren't issued as silver certificates, only gold certificates and Federal Reserve Notes. Please look at the bill's front to determine which you have; then check one of the following: "What is the value of a 1928 US 100 dollar Federal Reserve Note?" "What is the value of a 1928 US 100 dollar gold certificate?"
Not every old bill is automatically a certificate so it's important to check carefully. If it has a gold seal and says "Gold Certificate", see the question "What is the value of a 1928 US 10 dollar gold certificate?" for values. If it has a green seal and says "Federal Reserve Note", it's not a certificate, it's a Federal Reserve Note. See the question "What is the value of a 1928 US 10 dollar Federal Reserve Note?" for values.
That phrase appeared on all Federal Reserve Notes of the time because the US still issued gold-backed currency, even though FRNs were not gold certificates. Please see the question "What is the value of a 1928 US 5 dollar Federal Reserve Note?" for more information.
Please don't assume that every old bill has to be a silver certificate. 1928 $10 bills were issued as gold certificates with gold seals and as Federal Reserve Notes, with the familiar green seal and the words "Federal Reserve Note" across the top.Please check the bill's seal color and wording across your bill's top front; then see one of the following questions:"What is the value of a 1928 US 10 dollar Federal Reserve Note?""What is the value of a 1928 US 10 dollar gold certificate?"
The value of a 1928 twenty dollar bill that says "The Federal Reserve Missouri Bank of St. Louis" would actually depend on a number of things. The main factor that should be considered when determining value would be the condition of the bill.
All 1928 $5 Federal Reserve Notes carried the phrase "Redeemable in Gold", although they weren't gold certificates and in fact no gold certificates with that denomination were printed. Please see the question "What is the value of a 1928 A US 5 dollar Federal Reserve Note?" for more information.