Experiments were designed to determine the nature of amphotericin B (AMB)-5-fluorocytosine (5-FC) synergism against strains of Candida albicans susceptible to either drug alone. AMB strongly inhibited cell uptake of 14C-labeled 5-FC during incubation in a synthetic liquid medium. After several hours, however, this inhibition was dramatically released. Onset of 14C-5-FC uptake correlated with the known kinetics of AMB decay. These findings support the concept of sequential drug action proposed earlier. AMB apparently acts first and essentially alone until it approaches depletion. Experiments with a 5-FC-resistant mutant indicated that AMB interferes with transport of 5-FC to the cell interior.