Amino acid esters can destroy intracellular as well as isolated amastigotes of Leishmania mexicana amazonensis. In the present study we examined, using a tetrazolium reduction assay, the toxicity of the esters for amastigotes isolated from mouse lesions. Parasite killing by the “prototype” compound L-leucine methyl ester at 1 mM concentration and at pH 7.3 took place within 15–30 min. Time-lapse cinematographic observations showed that the amastigotes rounded up and became less phase-dense before they rapidly broke down. Ammonium chloride, ethylamine or monensin, known to raise the intracellular pH, reduced the sensitivity of the amastigotes to L-Leu-OMe. This finding suggests that an acidified compartment is involved in the destruction of the parasities. The leishmanicidal activity of a series of L-amino acid esters was also investigated. The ED50 (concentration for half maximal effect) for methyl esters was (in mM): Leu (0.62), Trp (0.96), Met (1.13), Glu (2.0), Phe (2.5), and Tyr (3.8). In contrast, the methyl esters of Ile, Val, Ala, βAla, Gly, Ser, His, and Pro were either inactive or weakly active at 15 mM. Benzyl esters were more active than their methyl homologs: the ED50 of the benzyl esters of Leu, Val, Ile, Gly, Ala, βAla, and Pro were, respectively, 0.07, 0.20, 0.22, 0.88, 1.5, 2.3, and 6.7 mM. Ranks of leishmanicidal activity may reflect differences in the rates of ester uptake and trapping by the amastigotes, in the specificity of the relevant hydrolytic enzyme(s), in the accumulation and metabolic fate of the released amino acids, or in the toxicity of the amino acid or alcohol released within the amastigotes.