A broth macrodilution method, performed as recommended by the National Committee for Clinical Laboratory Standards, was used for comparative testing of the new echinocandin antifungal agent MK-0991 and fluconazole against 50 yeast isolates belonging to 12 species of Candida. MK-0991 was shown to be highly effective against both fluconazole-susceptible and -resistant Candida spp., yielding minimum inhibitory concentrations ranging from < or = 0.19 to 0.78 microg/ml. Fungicidal activity was exerted at < or = 1.5 microg/ml for 73% of the isolates tested. This study suggests that MK-0991 has significant potential for clinical development.