We investigated the role of 27 disease-relevant Escherichia coli strains isolated from humans in the induction of histamine release from rat peritoneal mast cells and human basophilic granulocytes. Our data indicated that only the hemolysin-positive (HLY+) bacteria and the hemolysin-positive culture supernatants induced histamine release. For the latter, the hemolysin activity determined the degree of histamine secretion. Incubation of the target cells with washed HLY+ bacteria revealed a different secretory response. For the rat mast cells, histamine release paralleled expression of hemolysin activity, with the exception of strain S98 (O75:K5:H- HLY+), which induced less histamine, although its hemolysin activity was relatively high. No correlation between histamine secretion and hemolysin activity was observed when human basophils were stimulated with the HLY+ bacteria. Large amounts of histamine were still released, even when the hemolysin activity declined to zero. Our results support the potent role of the E. coli hemolysin as a pathogenicity factor in bacterial infection.