Twenty-six patients undergoing elective gastrointestinal surgery received a 1 g intravenous dose of cefpirome before operation. Serum and peritoneal fluid samples, obtained 0.5-7.6 h following administration, were assayed for cefpirome by a microbiological assay. The serum half-life of cefpirome was 2.1 h. The mean concentration of cefpirome in peritoneal fluid 0-2 h after administration was 44.4 micrograms/ml. The half-life of cefpirome in peritoneal fluid was 2 h, with mean concentrations of less than 10 micrograms/ml measured 6 h after administration. The mean percentage of intraperitoneal penetration of cefpirome over the study period was 97.7%. The therapeutic implications are discussed.