A respiratory-deficient mutant of Candida albicans MEN was generated by culturing cells in medium supplemented with ethidium bromide at 37 degreesC for 5 days. The respiratory-deficient mutant (C. albicans MMU11) was incapable of growth on glycerol, had a reduced oxygen uptake rate and demonstrated an altered mitochondrial cytochrome profile. Respiratory-competent cybrids were formed by mitochondrial transfer following fusion of protoplasts with those of C. albicans ATCC 44990. Mutant MMU11 possessed lower levels of ergosterol than the parental isolates and the cybrids, and demonstrated a small but statistically significant increase in tolerance to amphotericin B. The results demonstrated that disruption of mitochondrial function in C. albicans increases the tolerance to amphotericin B, possibly mediated by a reduction in cellular ergosterol content.