Eight strains of Bordetella pertussis labeled as "phase IV" and received from different laboratories were examined for their cultural, biochemical, and immunological characteristics in comparison with four phase I strains. Five of the strains possessed cultural and biochemical characteristics agreeing with published information on phase IV B. pertussis. Two of the strains had properties more closely resembling those of Brucella melitensis and one resembled Lophomonas sp.None of the phase IV strains possessed the protective antigen (PA) or the histamine-sensitizing factor (HSF) and they showed little immunological relationship with one another when tested for agglutination by antisera to phase IV except for L-46, L-51, and L-84, which formed a homogeneous serological group. Certain phase I antisera showed some degree of cross-reactivity with phase IV strains but there was much variation between the individual phase IV strains. Conversely the phase IV antisera cross-reacted with phase I organisms.