The term was published in the American Chemical Society's Chemical Abstracts in 1972, and is considered by some to be the longest real word. It does hold the record for the longest word in an English language publication in a serious context, that is, for some reason other than to publish a very long word.
In its complete form, the word is:
acetylseryltyrosylserylisoleucylthreonylserylprolylserylglutaminyl- phenylalanylvalylphenylalanylleucylserylserylvalyltryptophylalanyl- aspartylprolylisoleucylglutamylleucylleucylasparaginylvalylcysteinyl- threonylserylserylleucylglycylasparaginylglutaminylphenylalanyl- glutaminylthreonylglutaminylglutaminylalanylarginylthreonylthreonyl- glutaminylvalylglutaminylglutaminylphenylalanylserylglutaminylvalyl- tryptophyllysylprolylphenylalanylprolylglutaminylserylthreonylvalyl- arginylphenylalanylprolylglycylaspartylvalyltyrosyllysylvalyltyrosyl- arginyltyrosylasparaginylalanylvalylleucylaspartylprolylleucylisoleucyl- threonylalanylleucylleucylglycylthreonylphenylalanylaspartylthreonyl- arginylasparaginylarginylisoleucylisoleucylglutamylvalylglutamyl- asparaginylglutaminylglutaminylserylprolylthreonylthreonylalanylglutamyl- threonylleucylaspartylalanylthreonylarginylarginylvalylaspartylaspartyl- alanylthreonylvalylalanylisoleucylarginylserylalanylasparaginylisoleucyl- asparaginylleucylvalylasparaginylglutamylleucylvalylarginylglycyl- threonylglycylleucyltyrosylasparaginylglutaminylasparaginylthreonyl- phenylalanylglutamylserylmethionylserylglycylleucylvalyltryptophyl- threonylserylalanylprolylalanylserine