Minimal bactericidal concentrations of the aminoglycoside antibiotics amikacin, gentamicin, and netilmicin killed intraphagolysosomal test bacteria of selected assay strains of Serratia marcescens, though not as efficiently as rifampin. The system employed consisted of 55 vol % of fresh defibrinated human blood treated with 2 mg/ml of phenylbutazone which permitted ingestion of bacteria, but selectively inhibited microbicidal activity of the peripheral blood leukocytes. Extraphagocytic bacteria were killed with the aid of group A (phage tail) bacteriocins of S. marcescens. Inhibitory and subinhibitory concentrations of amikacin, gentamicin, and netilmicin combined with 55 vol % of defibrinated blood, respectively, yielded additive effects against all test strains of S. marcescens utilized and against Escherichia coli control strain ATCC 25922.