The urinary excretion of alanine aminopeptidase was measured in healthy volunteers, 59 receiving cephalosporins and 78 receiving aminoglycosides over a period of 3 days. A rise in excretion of the enzyme followed administration of cephacetrile 8.0 g daily but was not seen after 8.0 g cephalothin or 6.0 g doses of cefamandole or cephacetril. All the aminoglycosides tested produced a rise in enzyme excretion in the urine with amikacin (10 mg/kg/day), gentamicin (3 mg/kg/day) and sisomicin (3 mg/kg/day) producing higher mean MICs than seen with tobramycin (3 mg/kg/day) or netilmicin (3 mg/kg/day). The effect was shown to be dose-related in further studies with amikacin and netilmicin.