The flow- cytofluorometric susceptibility test (FCST) was incorporated into two in vitro synergy assays of amphotericin B-drug combinations. The FCST checkerboard assay and the FCST concentration-effect assay were developed to detect subtle modulation of amphotericin B fungicidal effects on Candida albicans and Cryptococcus neoformans. Amphotericin B x cyclosporine A was a synergistic fungicidal combination against both C. albicans and C. neoformans. Amphotericin B x gentamicin and amphotericin B x ketoconazole were synergistic combinations against C. neoformans. In all cases tested, the synergy was effective when 0.50-0.62-mu-g amphotericin B/ml was used with greater-than-or-equal-to 0.50-mu-g of the other drug/ml. The fungicidal effect of greater-than-or-equal-to 1.00-mu-g amphotericin B/ml overwhelmed the synergistic effects. A number of other drug combinations were additive. autonomous, or antagonistic.