Several different methods of performing susceptibility tests on six clinical isolates of Aspergillus are described. Some of the conditions that affected the level of susceptibility to drugs were: the type of media used, temperature and time of incubation, and the initial inoculum size. For amphotericin B susceptibility testing, the effectiveness of the polyene antibiotic as measured by visual growth was equivalent to the effectiveness as measured by inhibition of ribonucleic acid synthesis and dry-weight increase. For 5-fluorocytosine and rifampin, the visual-turbidity method gave minimum inhibitory concentrations that were much higher than those determined by effects on ribonucleic acid synthesis and dry weight. The reason for these discrepancies in susceptibility testing with 5-fluorocytosine and rifampin are unknown. We conclude that the most relevant test of this fungus to antifungal agents will have to be determined by the correlation of in vitro data with animal experiments and clinical results.