The pharmacokinetics of Cefoxitin was studied in 8 cirrhotic patients with ascites after i.v. administration of a single 30 mg/kg dose. Concentrations of cefoxitin in serum and in ascitic fluid were determined simultaneously and by a microbiologic plate diffusion method. The antibiotic followed a two-compartment open kinetic model. In ascitic fluid, Cefoxitin reached its maximum concentration of 32.80 +/- 13,78 micrograms/ml 2 h after administration. The mean elimination constant from ascitic fluid was 0.201 +/- 0.008 h(-1), significantly lower (p less than 0.05) than the slow disposition phase constant (beta = 0.556 +/- 0.17 h(-1)). At the dose studied and with a dosage interval of 8 h, the level of antibiotic in the ascitic fluid would be maintained at a value greater than the MIC of most cefoxitin-sensitive organisms.