A leukotoxic and hemolytic toxin was purified from cultures of Enterobacter cloacae. Stimulation of oxidative stress was observed and the production of reactive oxidant species was measured in leukocytes treated with toxin by means of nitroblue tetrazolium and chemiluminescence assays. Molecular weight of toxin was estimated by chromatography and SDS-PAGE. Two protean peaks with toxic activity were found in Sephadex G-100 (P1, 42.0 kDa; and P2, 13.3 kDa), The relative amounts between the peaks (P1/P2 = 0.36) changed when g-mercaptoethanol was employed (P1/P2 = 0.59), When Sephadex G-200 chromatography was performed, a protean peak of Ve = 113 mt (100 kDa) was found; its was dissociated with 3 M urea in toxic proteins of lower mass: 42, 27, and 13.3 kDa, SDS-PAGE (15%) showed a single toxin band of purified monomer (13.3 kDa), but electrophoresis of a 42-kDa toxin with urea presented three bands of trimer, dimer, and monomer. An increase of casein hydrolysate and albumin molecular weight was observed by chromatography after incubation with toxin due to the binding of both proteins with toxin,