Concentration-vs.-time curves in serum, peritoneal fluid, bone, and tissue fluid were determined after 2-g intravenous doses of moxalactam. Serum concentrations were measured in five volunteers (group 1; mean age, 22.8 years). Serum and peritoneal fluid concentrations were measured in eight patients (group 2; mean age, 66.5 years). Serum, bone, and tissue fluid concentrations were measured in eight patients (group 3; mean age, 71.5 years). The serum concentrations fitted an open two-compartment model. In groups 1, 2, and 3, the respective serum beta-phase half lives were 134, 169, and 184 min. In group 2, the peak concentration in peritoneal fluid was 43.8 mg/liter after 2.02 hr, with one-quarter of this concentration being maintained for 13 hr. In group 3, the bone concentrations were 19.4 mg/liter after 60 min and 19.1 mg/liter after 120 min; the peak concentration in tissue fluid was 36.1 mg/liter after 2.4 hr, with one-quarter of this concentration being maintained for 15 hr. These concentrations in tissues will inhibit most aerobic and anaerobic bacteria that cause infections.