A microtiter method for the determination of fungal sensitivities was used to determine the minimal inhibitory and fungicidal concentrations of two antifungal agents, amphotericin B and natamycin, both alone and in combination with four different antibiotics: rifampin, gentamicin, clindamycin, and tetracycline. Synergism was defined as a fourfold or greater reduction in the minimal inhibitory concentration, minimal fungicidal concentration, or both, of the antifungal agent in the presence of antibiotic; antagonism was defined as fourfold or greater increase in the minimal inhibitory concentration, minimal fungicidal concentration, or both. Amphotericin B and rifampin were synergistic against the majority of organisms, but synergism between amphotericin B and other antibiotics was infrequent. Combinations of natamycin and rifampin or of natamycin and gentamicin were synergistic against the majority of Fusarium solani tested. Combinations of amphotericin B and tetracycline were antagonistic against 14% of the organisms.