What is Medicare B?
Part B helps cover medically-necessary services like doctors' services, outpatient care, and other medical services. Part B also covers some preventive services. You can find out if you have Part B by looking at your Medicare card. For more details: http://www.medicare.gov/Publications/Pubs/pdf/10050.pdf
The Medicare Modernization Act of 2003 (MMA) provided for a special enrollment period for Medicare Part B for TRICARE beneficiaries who have Medicare Part A. If you have Medicare Part A (which generally pays for hospital care) and don't have Medicare Part B (which generally pays for doctor visits) you may be able to enroll in Part B without paying a late enrollment penalty.