N-Carbobenzoxy L-phenylalanine, glycine, L-leucine, and L-proline derivatives, their vinyl esters, and their 1,2-dibromoethyl esters were tested for antifertility activity in mice. Intraperitoneal administration reduced the pregnancy percentage and the number of fetuses per litter. Intravaginal administration reduced the pregnancy percentage significantly, with N-carbobenzoxyglycine vinly ester, N-carbobenzoxyglycine-1,2-dibromoethyl ester, N-carbobenzoxyl-L-leucine-1,2-dibromoethyl ester, and N-carbobenzoxy-L-proline-1,2-dibromoethyl ester producing 100% inhibition at 10 mg/kg/day. Sperm enzyme hydrolysis of the nonspecific substrate azocasein was inhibited significantly by certain N-carbobenzoxy amino acid esters in vitro. Specific substrate N-benzoyl-L-arginine ethyl ester hydrolysis was also inhibited. Compounds that inhibited N-benzoyl-L-arginine ethyl ester hydrolysis also demonstrated in vivo intravaginal antifertility activity.