ACETYL SERYL TYROSYL SERYL ISO LEUCYL THREONYL SERYL PROLYL SERYL­GLUTAMINYL PHENYL ALANYL VALYL PHENYL ALANYL LEUCYL SERYL SERYL VALYL­TRYPTOPHYL ALANYL ASPARTYL PROLYL ISOLEUCYL GLUTAMYL LEUCYL LEUCYL­ASPARAGINYL VALYL CYSTEINYL THREONYL SERYL SERYL LEUCYL GLYCYL­ASPARAGINYL GLUTAMINYL PHENYL ALANYL GLUTAMINYL THREONYL GLUTAMINYL­GLUTAMINYL ALANYL ARGINYL THREONYL THREONYL GLUTAMINYL VALYL­GLUTAMINYL GLUTAMINYL PHENYL ALANYL SERYL GLUTAMINYL VALYL TRYPTOPHYL­LYSYL PROLYL PHENYL ALANYL PROLYL GLUTAMINYL SERYL THREONYL VALYL­ARGINYL PHENYL ALANYL PROLYL GLYCYL ASPARTYL VALYL TYROSYL LYSYL VALYL­TYROSYL ARGINYL TYROSYL ASPARAGINYL ALANYL VALYL LEUCYL ASPARTYL­PROLYL LEUCYL ISOLEUCYL THREONYL ALANYL LEUCYL LEUCYL GLYCYL THREONYL­PHENYL ALANYL ASPARTYL THREONYL ARGINYL ASPARAGINYL ARGINYL ISOLEUCYL­ISOLEUCYL GLUTAMYL VALYL GLUTAMYL ASPARAGINYL GLUTAMINYL GLUTAMINYL­SERYL PROLYL THREONYL THREONYL ALANYL GLUTAMYL THREONYL LEUCYL­ASPARTYL ALANYL THREONYL ARGINYL ARGINYL VALYL ASPARTYL ASPARTYL­ALANYL THREONYL VALYL ALANYL ISOLEUCYL ARGINYL SERYL ALANYL ASPARAGINYL­ISOLEUCYL ASPARAGINYL LEUCYL VALYL ASPARAGINYL GLUTAMYL LEUCYL VALYL­ARGINYL GLYCYL THREONYL GLYCYL LEUCYL TYROSYL ASPARAGINYL GLUTAMINYL­ASPARAGINYL THREONYL PHENYL ALANYL GLUTAMYL SERYL METHIONYL SERYL­GLYCYL LEUCYL VALYL TRYPTOPHYL THREONYL SERYL ALANYL PROLYL ALANYL­SERINE = Tobacco Mosaic Virus, Dahlemense Strain.

This word has appeared in the American Chemical Society's Chemical Abstracts and is thus considered by some to be the longest real word.

ACETYL SERYL TYROSYL SERYL ISO LEUCYL THREONYL SERYL PROLYL SERYL­GLUTAMINYL PHENYL ALANYL VALYL PHENYL ALANYL LEUCYL SERYL SERYL VALYL­TRYPTOPHYL ALANYL ASPARTYL PROLYL ISOLEUCYL GLUTAMYL LEUCYL LEUCYL­ASPARAGINYL VALYL CYSTEINYL THREONYL SERYL SERYL LEUCYL GLYCYL­ASPARAGINYL GLUTAMINYL PHENYL ALANYL GLUTAMINYL THREONYL GLUTAMINYL­GLUTAMINYL ALANYL ARGINYL THREONYL THREONYL GLUTAMINYL VALYL­GLUTAMINYL GLUTAMINYL PHENYL ALANYL SERYL GLUTAMINYL VALYL TRYPTOPHYL­LYSYL PROLYL PHENYL ALANYL PROLYL GLUTAMINYL SERYL THREONYL VALYL­ARGINYL PHENYL ALANYL PROLYL GLYCYL ASPARTYL VALYL TYROSYL LYSYL VALYL­TYROSYL ARGINYL TYROSYL ASPARAGINYL ALANYL VALYL LEUCYL ASPARTYL­PROLYL LEUCYL ISOLEUCYL THREONYL ALANYL LEUCYL LEUCYL GLYCYL THREONYL­PHENYL ALANYL ASPARTYL THREONYL ARGINYL ASPARAGINYL ARGINYL ISOLEUCYL­ISOLEUCYL GLUTAMYL VALYL GLUTAMYL ASPARAGINYL GLUTAMINYL GLUTAMINYL­SERYL PROLYL THREONYL THREONYL ALANYL GLUTAMYL THREONYL LEUCYL­ASPARTYL ALANYL THREONYL ARGINYL ARGINYL VALYL ASPARTYL ASPARTYL­ALANYL THREONYL VALYL ALANYL ISOLEUCYL ARGINYL SERYL ALANYL ASPARAGINYL­ISOLEUCYL ASPARAGINYL LEUCYL VALYL ASPARAGINYL GLUTAMYL LEUCYL VALYL­ARGINYL GLYCYL THREONYL GLYCYL LEUCYL TYROSYL ASPARAGINYL GLUTAMINYL­ASPARAGINYL THREONYL PHENYL ALANYL GLUTAMYL SERYL METHIONYL SERYL­GLYCYL LEUCYL VALYL TRYPTOPHYLTHREONYL SERYL ALANYL PROLYL ALANYL­SERINE = Tobacco Mosaic Virus, Dahlemense Strain.

This word has appeared in the American Chemical Society's Chemical Abstracts and is thus considered by some to be the longest real word.

I suppose that you think to tobacco mosaic virus, but the chemical name is longer.

See the link below.