The biliary excretions of latamoxef and N-methyltetrazolethiol (NMTT) were studied in patients with T-tubes inserted in their common bile ducts, and in bile fistulated male rats. The highest concentrations of latamoxef and NMTT in 6 patients with cholelithiasis were obtained 1-2 h after intravenous injection of a single 1-g dose, and the mean peak values were 66.2 and 0.85 microgram/ml, respectively. The values in patients with gallbladder carcinoma or pancreas head carcinoma were much lower than those with cholelithiasis. The biliary excretions of latamoxef and NMTT in rats were much higher and faster than in humans, with the highest levels being obtained within 30 min after injection. About 20% of the injected dose was recovered as latamoxef and 4% as decomposed materials in 2.5 h after injection. Latamoxef was not decomposed in human or rat bile when kept at -20 degrees C for 4 weeks if the pH was adjusted to 6; some decomposition occurred (10-20%) if the pH was not adjusted.