The degranulation and myeloperoxidase-H2O2-halide activities of human polymorphonuclear leukocytes from healthy donors were tested after co-incubation with either Brucella melitensis 16M, Staphylococcus aureus or Staphylococcus aureus in presence of lipopolysaccharide, protein fraction, native hapten and soluble fractions released at 65 degrees C from smooth strain of Brucella melitensis 16M. The degranulation and myeloperoxidase activities of polymorphonuclear leukocytes were significantly higher when co-incubated with Staphylococcus aureus than with Brucella melitensis. The presence of lipopolysaccharide, protein fraction, and native hapten did not cause significant modification of either degranulation or myeloperoxidase activities of polymorphonuclear leukocytes against Staphylococcus aureus. Soluble fraction released at 65 degrees C produced a significant reduction in the myeloperoxidase activity but did not alter the degranulation of polymorphonuclear leukocytes triggered by Staphylococcus aureus.