Cephamandol 6.0 g, cephazolin 6.0 g or cephacetrile or cephalothin 8.0 g were administered as short-term infusions on 3 consecutive days to informed volunteers, who had no history or evidence of impairment of renal function. There were 15 subjects in the cephamandol, cephacetrile and cephalothin groups and 14 subjects in the cephazolin group. Alanine-aminopeptidase, a characteristic tubule enzyme, was determined in a 24-hour urine 2 days before administration, during the 3 day administration and on the 4 subsequent days. In addition, alanine-aminopeptidase was also estimated immunologically in concentrated urine with the aid of an anti-brush border antibody. Cephamandol, cephazolin and cephalothin were completely without effect on the proximal tubule. Cephacetrile, on the other hand, showed clear reactions in 9 out of 15 subjects, in the form of elevated AAP activity in urine and in 6 of the cases membrane elimination was demonstrable immunologically. After withdrawal of the medication, the values of the responder group returned spontaneously to normal, i.e. no cumulative effect was detected. These investigations show that elimination of alanine-aminopeptidase in the urine is a very sensitive index of the action of cephalosporins on renal tubules.