The relative efficacies of free amphotericin B (Amp B) and liposome-encapsulated Amp B (L-AmpB) in the treatment of established Candida albicans infection in mice rendered neutropenic with cyclophosphamide were studied. AmpB was entrapped in multilamellar liposomes composed of dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylglycerol in a molar ratio of 7:3. Infected mice treated with single doses of 3 mg L-AmpB/kg of body weight had an increased survival time compared with those injected with either single (dose, 0.8 mg/kg) or multiple doses (dose, 0.8 mg/kg daily for five days) of free AmpB. When treatment was delayed beyond three days postinfection, neither single nor multiple doses of free AmpB resulted in increased survival, whereas treatment with single-dose L-AmpB (dose, 4 mg/kg) showed efficacy when delayed as much as four days postinfection. Five days postinfection only higher doses (dose, 5.6 mg-11.2 mg/kg) of L-AmpB improved survival time and the renal impairment present in the infected animals. These data provided a rational basis for using high-dose L-AmpB to treat fungal diseases in humans, particularly in neutropenic patients.