The in vitro activity of amphotericin B and 5-fluorocytosine against 40 isolates of different species of Candida was studied. Synergism was seen in 35 of 40 organisms studied (88%) with achievable serum concentrations of the antibiotics. Synergism was striking in many strains that were resistant to 5-fluorocytosine. When the minimal inhibitory concentration of 5-fluorocytosine was greater than 400 mug/ml, synergism was seen in 18 of 20 isolates of Candida (90%).