A high-performance liquid chromatographic method has been developed which enables accurate determination of a new synthetic antibacterial agent, AT-2266, and its metabolite, M-2, in plasma, and AT-2266 and its five metabolites, M-1, M-2, M-3, M-4 and M-5, in urine. AT-2266 is extracted as ethyl carbamate with chloroform containing 1% ethyl chloroformate and assayed on a liquid chromatograph equipped with an ultraviolet detector at 340 nm. Accurate determinations are possible over a concentration range of 0.1-10 micrograms/ml AT-2266 in plasma, and 1-500 micrograms/ml AT-2266 in urine. The coefficient of variation at the 2 micrograms/ml level of AT-2266 is 1.9% (n = 6). The minimum detectable concentrations of AT-2266 in plasma and urine are 0.01 micrograms/ml and 0.1 micrograms/ml, respectively, and those of other metabolites are similar to those of AT-2266. Plasma levels and urinary excretion of AT-2266 in a man following single oral administration (400 mg) have also been determined.