Adenylate cyclase (AC) toxin from B. pertussis enters eukaryotic cells where it produces supraphysiologic levels of cAMP. Purification of AC toxin activity [(1989) J. Biol. Chem. 264, 19279] results in increasing potency of hemolytic activity and electroelution of the 216-kDa holotoxin yields a single protein with AC enzymatic, toxin and hemolytic activities. AC toxin and E. coli hemolysin, which have DNA sequence homology [(1988) EMBO J. 7, 3997] are immunologically cross-reactive. The time courses of hemolysis elicited by the two molecules are strikingly different, however, with AC toxin eliciting cAMP accumulation with rapid onset, but hemolysis with a lag of greater than or equal to 45 min. Finally, osmotic protection experiments indicate that the size of the putative pore produced by AC toxin is 3-5-fold smaller than that of E. coli hemolysin.