Neutrophils represent one of the host's primary defenses against invading organisms. These cells often arrive at the site of infection prior to macrophages (M phi). Neutrophils release myeloperoxidase (MPO) into the micro-environment during phagocytosis. Previous studies by the present investigators have shown that M phi bactericidal activity is enhanced by exposure to MPO. A recent report suggests that as much as 40% of this protein is enzymatically inactive once it is released into the micro-environment. In the present study, exposure of M phi to an enzymatically inactive form of MPO (iMPO) or another mannosylated protein, mannosylated bovine serum albumin (mBSA), can induce the same enhanced Mø-mediated bacterial cell killing observed with the active form of MPO. Furthermore, this phenomenon is limited as galactosylated BSA (gBSA) did not induce enhancement of bacterial killing. The data suggest that interaction of either enzymatically active or inactive mannosylated proteins with the M phi mannose receptor (MMR), is sufficient to enhance M phi bactericidal activity and further underscores the binding of the MMR and resultant responses as a major host defense mechanism.