The pharmacology of cephradine, cephalexin and a new oral cephalosporin, cefaclor, has been compared in six volunteers. Cefaclor was absorbed rapidly and was cleared from the serum more rapidly than the other two agents. This was probably partially due to its instability in serum at body temperature, which was investigated. Against a wide range of common pathogens cefador was the more active oral cephalosporin. In particular the activity against Neisserla gonorrhoeae and Haemophilus influenzae was of interest.