The kinetics of the R and S epimers of moxalactam were followed after single intravenous doses of 50 mg/kg to 12 patients, 8 to 45 mo old, with cellulitis or epiglottitis. High-pressure liquid chromatography was used to determine serum concentrations. Total body clearance and apparent volume of distribution of the R epimer were higher than those of the S epimer (P less than 0.004). Total body clearance of R, S, and R + S moxalactam ranged from 29.01 to 183.7, 19.00 to 79.95, and 23.34 to 113.6 ml/min/m2. Mean elimination half-lives of R, S, and R + X moxalactam were 2.04, 2.26, and 2.24 hr. The higher incidence of the more active R epimer and the 500% interpatient variation in clearance may indicate a need for monitoring serum and cerebrospinal fluid concentrations in patients with severe unresponsive central nervous system infectious treated with moxalactam.