The in-vitro activity of cilofungin, a derivative of echinocandin B, was compared with that of amphotericin B in Sabouraud dextrose and Antibiotic Medium No. 3 against 100 clinical isolates of yeasts. Cilofungin appeared to be as effective, as amphotericin B against Candida albicans and yet more effective against Can. tropicalis as far as growth inhibition was concerned. Cilofungin was less active than amphotericin B against Can. (Torulopsis) glabrata and other species of Candida. It was not active against Cryptococcus neoformans. Minimum fungicidal concentrations (MFCs) of cilofungin were highly dependent on the medium, especially with Can. albicans. Low MFCs were observed in Antibiotic Medium 3 and very high MFCs were measured in Sabouraud's medium. Using a killing curve method, the initial rate of killing of amphotericin B was proportional to concentration with Can. albicans, Can. tropicalis, and Can. glabrata. With cilofungin the rate of killing was proportional to concentration only for Can. tropicalis.