Bactericidal activity in serum produced after administration of 1-g intravenous doses of cefoperazone, cefotaxime, ceftizoxime, and moxalactam was ascertained in six healthy subjects. The assay organisms were a strain of Staphylococcus aureus which was moderately susceptible to the drugs (MBC, 2 to 8 micrograms/ml) and an isolate of Escherichia coli which was highly susceptible (MBC, 0.08 to 0.3 microgram/ml). Drug concentrations and bactericidal titers were measured from samples taken for up to 12 h after the dose. No bactericidal activity against the S. aureus strain was found at 4 to 6 h and beyond for any of the drugs. Ranking of the in vivo bactericidal activity of the drugs was cefoperazone = cefotaxime greater than ceftizoxime = moxalactam. Against the E. coli isolate, bactericidal activity was present for 8 h for cefotaxime, and for 12 h for the other drugs. Ranking of the drugs in terms of extent and duration of in vivo bactericidal activity versus E. coli was moxalactam = ceftizoxime greater than cefoperazone greater than cefotaxime. After administration of 1-g doses of these new beta-lactams, bactericidal activity in serum was maintained for 12 h against highly susceptible bacteria. More frequent (6 to 8 h) or higher (greater than or equal to 2 g) dosing appears to be necessary to achieve prolonged serum bactericidal activity against less susceptible isolates (MBC, greater than or equal to 2 to 8 micrograms/ml).