Mutants of Saccharomyces cerevisiae were isolated which have decreased ability to hydrolyze leucine beta-naphthylamide, a chromogenic substrate for amino-peptidases. The mutations were shown by starch gel electrophoresis to affect one of four different aminopeptidases. Mutations affecting a given enzyme belong to a single complementation group. The four genes were symbolized lap1, lap2, lap3, and lap4, and the corresponding enzymes LAPI, LAPII, LAPIII, and LAPIV. Both lap1 and lap4 were mapped to the left arm of chromosome XI, and lap3 was mapped to the left arm of chromosome XIV. Strains which possessed only one of the four leucine aminopeptidases (LAPs) were constructed. Crude extracts from these strains were used to study the properties of the individual enzymes. Dialysis against EDTA greatly reduced the activity of all the LAPs except for LAPIII. Of the cations tested, Co2+ was the most effective in restoring activity. LAPIV was the only LAP reactivated by Zn2+. LAPI was purified 331-fold and LAPII was purified 126-fold from cell homogenates. Both of the purified enzymes had strong activity on dipeptides and tripeptides. The activity levels of the LAPs are strongly dependent on growth stage in batch culture, with the highest levels in early-stationary phase. Strains lacking all four LAPs have slightly lower growth rates than wild-type strains. The ability of leucine auxotrophs to grow on dipeptides and tripeptides containing leucine is not impaired in strains lacking all four LAPs.