(1,185) ACETYLSERYLTYROSYLSERYLISOLEUCYLTHREONYLSERYLPROLYLSERYLGLUTAMINYLPHENYLALANYL
VALYLPHENYLALANYLLEUCYLSERYLSERYLVALYLTRYPTOPHYLALANYLASPARTYLPROLYLISOLEUCYLGLUTAMYL
LEUCYLLEUCYLASPARAGINYLVALYLCYSTEINYLTHREONYLSERYLSERYLLEUCYLGLYCYLASPARAGINYL
GLUTAMINYLPHENYLALANYLGLUTAMINYLTHREONYLGLUTAMINYLGLUTAMINYLALANYLARGINYLTHREONYL
THREONYLGLUTAMINYLVALYLGLUTAMINYLGLUTAMINYLPHENYLALANYLSERYLGLUTAMINYLVALYLTRYPTOPHYL
LYSYLPROLYLPHENYLALANYLPROLYLGLUTAMINYLSERYLTHREONYLVALYLARGINYLPHENYLALANYLPROLYL
GLYCYLASPARTYLVALYLTYROSYLLYSYLVALYLTYROSYLARGINYLTYROSYLASPARAGINYLALANYLVALYLLEUCYL
ASPARTYLPROLYLLEUCYLISOLEUCYLTHREONYLALANYLLEUCYLLEUCYLGLYCYLTHREONYLPHENYLALANYL
ASPARTYLTHREONYLARGINYLASPARAGINYLARGINYLISOLEUCYLISOLEUCYLGLUTAMYLVALYLGLUTAMYL
ASPARAGINYLGLUTAMINYLGLUTAMINYLSERYLPROLYLTHREONYLTHREONYLALANYLGLUTAMYLTHREONYL
LEUCYLASPARTYLALANYLTHREONYLARGINYLARGINYLVALYLASPARTYLASPARTYLALANYLTHREONYLVALYL
ALANYLISOLEUCYLARGINYLSERYLALANYLASPARAGINYLISOLEUCYLASPARAGINYLLEUCYLVALYLASPARAGINYL
GLUTAMYLLEUCYLVALYLARGINYLGLYCYLTHREONYLGLYCYLLEUCYLTYROSYLASPARAGINYLGLUTAMINYL
ASPARAGINYLTHREONYLPHENYLALANYLGLUTAMYLSERYLMETHIONYLSERYLGLYCYLLEUCYLVALYLTRYPTOPHYL
THREONYLSERYLALANYLPROLYLALANYLSERINE
= 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.
An evil amount of 1,185 letters! I got this at http://www.fun-with-words.com/word_longest.html