Amphotericin B in combination with 5-fluorocytosine was synergistic against three clinical isolates of Aspergillus fumigatus and one of three clinical isolates of A. flavus. Amphotericin B in combination with rifampin was synergistic against all six clinical isolates of Aspergillus tested. The levels of 5-fluorocytosine and rifampin required for synergism were higher than clinically achievable concentrations when measurements of synergism were based on visual turbidity; but when the effects of the drugs were measured by inhibition of ribonucleic acid synthesis or dry-weight increase, much lower concentrations were effective.