Minimal bactericidal concentrations of trimethoprim, nalidixic acid, and nitrofurantoin revealed effective intraphagolysosomal bactericidal activity against several assay strains of Serratia marcescens as determined with phenylbutazone-treated (2 mg/ml) fresh defibrinated human blood (55 vol%), following killing of extraphagocytic test bacteria with group A (phage tail) bacteriocins of S. marcescens. The degree of intraphagocytic killing activity of trimethoprim, nalidixic acid, and nitrofurantoin approximated that of rifampin. Inhibitory and subinhibitory concentrations of cotrimoxazole or trimethoprim combined with 55 vol% of defibrinated blood, respectively, yielded additive effects against all test strains of S. marcescens. However, combinations of nalidixic acid and nitrofurantoin with blood, respectively, resulted in essentially indifferent effects against S. marcescens.