Myeloperoxidase (MPO) is reported to selectively bind to bacteria. The present study provides direct evidence of MPO binding selectivity and tests the relationship of selective binding to selective killing. The microbicidal effectiveness of H(2)O(2) and of OCl(-) was compared to that of MPO plus H(2)O(2). Synergistic microbicidal action was investigated by combining Streptococcus sanguinis, a H(2)O(2)-producing microbe showing low MPO binding, with high-MPO-binding Escherichia coli, Staphylococcus aureus, or Pseudomonas aeruginosa without exogenous H(2)O(2), with and without MPO, and with and without erythrocytes (red blood cells [RBCs]). Selectivity of MPO microbicidal action was conventionally measured as the MPO MIC and minimal bactericidal concentration (MBC) for 82 bacteria including E. coli, P. aeruginosa, S. aureus, Enterococcus faecalis, Streptococcus pyogenes, Streptococcus agalactiae, and viridans streptococci. Both H(2)O(2) and OCl(-) destroyed RBCs at submicrobicidal concentrations. Nanomolar concentrations of MPO increased H(2)O(2) microbicidal action 1,000-fold. Streptococci plus MPO produced potent synergistic microbicidal action against all microbes tested, and RBCs caused only a small decrease in potency without erythrocyte damage. MPO directly killed H(2)O(2)-producing S. pyogenes but was ineffective against non-H(2)O(2)-producing E. faecalis. The MPO MICs and MBCs for E. coli, P. aeruginosa, and S. aureus were significantly lower than those for E. faecalis. The streptococcal studies showed much higher MIC/MBC results, but such testing required lysed horse blood-supplemented medium, thus preventing valid comparison of these results to those for the other microbes. E. faecalis MPO binding is reportedly weak compared to binding of E. coli, P. aeruginosa, and S. aureus but strong compared to binding of streptococci. Selective MPO binding results in selective killing.