Neither cefoperazone, cefotaxime, mezlocillin nor piperacillin significantly killed intraphagocytic bacteria of representative assay strains of Serratia marcescens, as determined with 55 vol% of fresh defibrinated human blood treated with 2 mg/ml of phenylbutazone, which permitted phagocytic ingestion of bacteria, but selectively inhibited the microbicidal activity of peripheral blood leukocytes. Extraphagocytic bacteria were killed with group A (phage tail) bacteriocins of S. marcescens. However, minimal bactericidal concentrations of fosfomycin displayed intraphagolysosomal bactericidal activity which approximated that of rifampin. Inhibitory and subinhibitory concentrations of cefoperazone, cefotaxime, fosfomycin, mezlocillin, and piperacillin combined with 55 vol% of defibrinated human blood, respectively, yielded additive effects against all test strains of S. marcescens utilized and Escherichia coli control strain ATCC 25922.