Few data are available to show how serum dilution test results correlate with results of antimicrobial assays and determinations of minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBCs). Serum dilution tests were performed for 65 patients with bacteremia from serious infections. Assays for antimicrobial agent levels and determinations fo MICs and MBCs against the infecting organisms were performed simultaneously. Mueller-Hinton broth and Mueller-Hinton broth supplemented with Ca++, Mg++, and pooled human serum were compared as diluents for the serum either medium yielded positive correlations, better correlations were found between measured and expected serum bactericidal activities when both ther serum dilution test dilutions and the MIC and MBC determinations were done in the supplemented MHB-human serum solution.