A case of Candida infection of the knee and probably also of the hip is reported, emphasizing the predisposing factors in its genesis. A treatment program using a bioassay of amphotericin B concentrations in the serum and joint aspirate is discussed. This allows use of a dose lower than usually recommended and avoids the more toxic dose-related side effects. Amphotericin B administered systemically penetrated the joint in therapeutic levels. Systemic treatment and repeated aspirations produced a prompt response obviating the need for surgical drainage.