Aeromonas hydrophila is a facultatively anaerobic, gram-negative rod which has been often regarded as an opportunistic pathogen in hosts with compromised defense mechanisms. A. hydrophila can produce septicemia with skin lesions such as necrosis, edema, and blisters. Therefore, it is necessary to investigate what kinds of exocellular ?oxic or pathogenic factors are produced by A. hydrophila for elucidation of pathogenesis and effective treatment. This study was designed to determine the extracellular enzymes produced by two strains of A. hydrophila which were isolated from the blood and wound of patients with septicemia and osteomyelitis, respectively. Histamine release from rat peritoneal mast cells evoked by A. hydrophila culture supernatant was assayed. The morphologica changes of rat peritoneal mast cells treated with A. hydrophila culture supernatant were observed by electron microscopy. A. hydrophila produced elastease, protease DNase, hemolysin(alpha and beta), lipase, lecithinase, and gelatinase but it did not produce coagulase, hyaluron dase, chondroitin sulfatase or mucinase. The culture supernatant fluid of A. hydrophila induced aistamine release from rat peritoneal mast cells. Transmission electron microscopic studies of mast cells treated with the culture supernatant revealed microvesicles of various size in the cytoplasm and around the cell membrane. These findings suggest that A. hydrophila culture supernatant induces histamine release from rat peritoneal mast cells by a process that may involve a resicular transport mechanism, namely, piecemeal secretion.