We have identified an adenylate cyclase toxin in urea extracts and culture supernatant fluids of Bordetella pertussis (2). The ability of this toxin and the lack of a strong correlation between its activity and adenylate cyclase activity found in urea extracts suggest that it is an oligomer of readily dissociable subunits. The mechanism by which Bordetella adenylate cyclase toxin interacts with target cells is unknown, but polyvalent cations are necessary. Neutrophils exposed to the toxin acquire a 39,000 Mr protein that can also be photoaffinity labeled with 32P-ATP. We anticipate that this protein will prove to be a catalytic component of Bordetella adenylate cyclase toxin. Susceptible cells exposed to Bordetella adenylate cyclase toxin are functionally aberrant. In phagocytes, decreased bactericidal capacity may be important in the pathogenesis of human whooping cough and other Bordetella infections occurring in domestic animals. The effects of the toxin on neoplastic cells may offer new insights into the factors controlling their growth and differentiation. Bordetella adenylate cyclase toxin is a unique bacterial product. Further purification and characterization of this toxin will add to our understanding of cell-protein interactions and pathogen-host relationships.