Mice immunized with formalin-killed Candida albicans were resistant to challenge by a lethal amount of viable C. albicans. The growth-inhibitory activity to C. albicans was detected in sera from the immunized mice, and was inhibited by the addition of anti-transferrin antibody or ferric sulfate. Both the amount of transferrin and the unsaturated iron-binding capacity (UIBC) in the serum were significantly increased, indicating that apo-transferrin increased in the immunized mice. Moreover, the intraperitoneal administration of apo-transferrin enhanced the protection from the Candida infection in vivo.