Chemical name of Dahlemense Strain of Tobacco Mosaic Virus is 3rd longest in English language, apparently (not sure I'd want to have a conversation with anyone who thinks this is really a valid English word, but anyway):

        acetylseryltyrosylserylisoleucylthreonylserylproly lserylglutaminyl-
        phenylalanylvalylphenylalanylleucylserylserylvalyl tryptophylalanyl-
        aspartylprolylisoleucylglutamylleucylleucylasparag inylvalylcysteinyl-
        threonylserylserylleucylglycylasparaginylglutaminy lphenylalanyl-
        glutaminylthreonylglutaminylglutaminylalanylarginy lthreonylthreonyl-
        glutaminylvalylglutaminylglutaminylphenylalanylser ylglutaminylvalyl-
        tryptophyllysylprolylphenylalanylprolylglutaminyls erylthreonylvalyl-
        arginylphenylalanylprolylglycylaspartylvalyltyrosy llysylvalyltyrosyl-
        arginyltyrosylasparaginylalanylvalylleucylaspartyl prolylleucylisoleucyl-
        threonylalanylleucylleucylglycylthreonylphenylalan ylaspartylthreonyl-
        arginylasparaginylarginylisoleucylisoleucylglutamy lvalylglutamyl-
        asparaginylglutaminylglutaminylserylprolylthreonyl threonylalanylglutamyl-
        threonylleucylaspartylalanylthreonylarginylarginyl valylaspartylaspartyl-
        alanylthreonylvalylalanylisoleucylarginylserylalan ylasparaginylisoleucyl-
        asparaginylleucylvalylasparaginylglutamylleucylval ylarginylglycyl-
        threonylglycylleucyltyrosylasparaginylglutaminylas paraginylthreonyl-
        phenylalanylglutamylserylmethionylserylglycylleucy lvalyltryptophyl-
        threonylserylalanylprolylalanylserine