Preliminary observations have shown that AmBisome, a liposomal formulation of amphotericin B (Vestar Inc.), is effective and non-toxic in animal and human visceral leishmaniasis. The activity of multiple doses of this drug on Leishmania infantum, in BALB/c mice was investigated, and amphotericin B concentration in liver and spleen was determined. Groups of infected mice were treated intravenously with 3, 5, or 7 doses of AmBisome (3 mg/kg) over 3, 10 and 25 days, respectively. The antileishmanial activity of the drug was compared with that of meglumine antimoniate (28 mg Sbv/kg per day over 21 days). Three consecutive daily doses of AmBisome were sufficient to clear all parasites from the liver of mice, while antimony did so only after 21 doses. Twenty-four-48 h after their last dose all the AmBisome-treated mice showed very high amphotericin B concentrations in liver (61.2-76.2 micrograms/g) and spleen (39.8-72.1 micrograms/g) with no overt signs of toxicity. Mice that received 2 or 4 doses at intervals of 5 to 8 days, maintained drug levels as high as those detected after 3 consecutive doses over 11 and 26 days, respectively. This should enable visceral leishmaniasis treatment on an intermittent or outpatient basis, thereby reducing overall treatment costs.