Nonopsonized Brucella abortus and bacteria treated with fresh antiserum, heat-inactivated antiserum, or normal bovine serum were evaluated for their ability to stimulate production of superoxide anion and myeloperoxidasemediated iodination by neutrophils from cattle. Brucella abortus opsonized with fresh antiserum or beat-inactivated antiserum stimulated production of approximately 3 nmol of 0 2 -/106 neutrophils/30 min. Similarly treated bacteria also stimulated the binding of approximately 4.3 nmol of Nal/107 neutrophils/h to protein. Significant CP < 0.05) production of 0 2 - and iodination activity by neutrophils were not stimulated by nonopsonized bacteria or organisms treated with normal bovine serum. Seemingly, B abortus stimulated oxidative metabolism in bovine neutrophils; however, the stimulation was dependent on the presence of bacterial usociated opsonins.