This study assessed the efficacy, toxicity, and pharmacology of low-dose amphotericin B given prophylactically to patients (serum concentrations of 0.2-0.4 microgram/ml) undergoing bone marrow transplantation. Yeast isolates from patients' oropharyngeal areas had MICs of 0.1-0.2 microgram/ml, and none were amphotericin B resistant. The effect of low-dose amphotericin B on reducing the numbers of yeast colonizing the oropharyngeal area was significant (P less than .01). The average delay in switching to high-dose prophylactic amphotericin B was only 1 day; the decision to do so because of a perceived fungal infection occurred more frequently for the placebo group (P = .06). Fewer patients from the low-dose amphotericin B group (8.8%) than from the placebo group (14.3%) had fungi isolated from normally sterile body sites (P = .35). Infusion-related side effects but not systemic toxicities were significantly greater (P less than .001) in the amphotericin B group. The 6-week survival was greater in those receiving amphotericin B (P less than .03), but the improved survival could not be attributed to the prevention of fungal infections.