One hundred yeasts were studied. Tests included detailed identification and determination of 24- and 48-hr minimal inhibitory concentrations and minimal fungicidal concentrations of 5-fluorocytosine (5-FC). Final identifications included 57 isolates of Candida albicans, 15 isolates of C. tropicalis, 13 isolates of C. parapsilosis, 7 isolates of Torulopsis glabrata, 3 isolates of C. guilliermondii, 2 isolates each of C. stellatoidea and Cryptococcus neoformans, and 1 isolate of Candida krusei. Twenty-three original identifications were in error; these involved mostly C. albicans, C. tropicalis, C. parapsilosis, and T. glabrata. Inhibitory end point readings based on 24 hr of incubation were misleading. Whereas 79 of 91 isolates of Candida appeared to be inhibited at 24 hr by 12.5 μg or less of 5-FC/ml, only 52 were inhibited at 48 hr; whereas only 12 isolates appeared to be resistant to 100 μg/ml after 24 hr, 37 were resistant after 48 hr. Susceptibility varied amount the different species. C. tropicalis was the most susceptible, with 10 of 15 isolates (66.7%) being inhibited by 12.5 μg or less/ml and 7 (46.7%) being killed by 100 μg or less/ml. C. albicans was similarily susceptible; 33 of 57 isolates (57.9%) were inhibited by 12.5 μg or less/ml and 25 (43.9%) were killed by 100 μg or less/ml. C. parapsilosis was quite resistant, as only 4 of 13 isolates (30.8%) were inhibited by 12.5 μg or less/ml and 3 (23.1%) were killed by 100 μg or less/ml.