Current monotherapy against visceral leishmaniasis has serious side effects, and resistant Leishmania strains have been identified. Amphotericin B (AmB) has shown an extraordinary antileishmanial efficacy without emergence of resistance; however, toxicity has limited its general use. Results obtained showed, using a fixed-ratio analysis, that the combination of diallyl thiosulfinate (allicin) and AmB ranged from moderately synergic to synergic at low concentrations (0.07 μM AmB plus 35.45 μM allicin induced 95% growth inhibition). None of the treatments, alone or in combination, had noticeable adverse effects on macrophages (M) in the concentration range examined (allicin, 0.5, 1, 5 and 10 μM; AmB, 0.05, 0.075, and 0.1 μM). Allicin, AmB, or the combination did not affect the infection rate (percentage of infected M) of Leishmania. Allicin enhanced the activity of AmB on intracellular amastigotes of Leishmania donovani and L. infantum (ca. 45% reduction of amastigote burden with 0.05 μM AmB plus 10 μM allicin); this represented nearly a 2-fold reduction in the 50% inhibitory concentration (IC50) of the antibiotic added alone. Results point toward the possible utility of testing this combination in vivo to reduce the toxicity associated with monotherapy with AmB.