The diagnosis of early, mild, or atypical whooping cough is dependent upon identification of the etiologic agent. Although procedures for the isolation of Bordetella pertussis1 have been available for many years and are of proven diagnostic aid, they have not gained the wide usage they seem to deserve. A recognized limitation is the time needed: 3, more often 4, and occasionally more, days are required for a positive report. The possibility of using fluorescent antibody methods for rapid identification of B. pertussis in nasopharyngeal specimens is suggested by the promising results reported in recent studies of a number of bacterial infections. For example, Group A streptococci,2,3 enteropathogenic Escherichia coli,4 Haemophilus influenzae, Type B,5 Neisseria gonorrhoeae,6 Vibrio comma,7 Leptospira,8 and certain other bacterial agents have been demonstrated by means of this technique in specimens taken directly from the patient. The work of de Repentigny and Frappier,9 although not concerned with diagnosis, indicated that specific antigens of B. pertussis could be stained with dye-conjugated antibody. In a recent diagnostic study by Donaldson and Whitaker,10 31 of 36 nasopharyngeal specimens from infants with symptoms of clinical whooping cough were found positive with the fluorescent antibody technique ; cultural results were not given.In the present study, preliminary tests with an available pertussis antiserum conju¬ gated with fluorescein isothiocyanate showed brilliant fluorescence in slide preparations of several strains of B. pertussis; fluorescence was not observed with a number of cultures