Cephalexin levels in the ejaculates of 31 infection-free volunteers have been determined following the administration of 1 g of antibiotic. In 16 subjects the cephalexin was administered by intramuscular injection and in 15 by mouth. In each case the volunteers were divided into groups of five each of which received the drug, 1, 2 or 4 hours before the collection of semen. Antibiotic concentrations were determined microbiologically on agar plates using Sarcina lutea ATCC 9341. For selection purposes healthy subjects were considered to be those showing a normal anamnesis and physical exploration, an absence of leucocytes demonstrable by a cytomorphological evaluation of the semen, semen citric acid and fructose levels within normal limits and a qualitatively and quantitatively similar saprophytic flora to that seen in a culture of urine taken immediately before the collection of semen. Maximum concentrations were obtained 2 hours after the administration of the antibiotic and the concentration following intramuscular injection (6.9 microgram/ml) was significantly greater than that following oral administration (4.7 microgram/ml).