X-63, a polyene antibiotic of heptaene series showed activity against experimental infection with Candida albicans in mice, by several routes. The oral, intraperitoneal and intravenous activities ranged between dosages of 3.1–50, 0.06–0.5 and 0.25–0.5 mg/kg, respectively. Seven days treatment by intraperitoneal route caused significant progressive reduction of colony-forming units in kidney homogenates of infected mice, which was comparable to amphotericin B. The regression of infection was further demonstrated histopathologically. In vitro, antibiotic X-63 was similar to nystatin against nine strains of Candida used. It also proved active against amphotericin B-resistant strains of C. albicans, C. tropicalis, C. pseudotropicalis, C. krusei, and C. parakrusei. The use of combined drug therapy in candidiasis has been discussed.