The prophylactic effects of a Basidiomycetes preparation, AHCC, against lethal Candida albicans infection were investigated in non treated or immunosuppressed mice. In the cyclophosphamide-or 5-fluorouracil (5-FU)-treated leukopenic mice and nontreated mice, the intraperitoneal administration of AHCC prior to C. albicans infection clearly prolonged the survival periods of the infected mice. In doxorubicin-treated mice, AHCC was less but significantly effective. On the other hand, in prednisolone-treated mice, AHCC was not effective. Oral administration of AHCC also protected the 5-FU-treated mice from lethal Candida infection, as indicated by prolongation of the survival periods and inhibition of Candida growth in the kidneys of these mice and by the increase in a number of neutrophils in their peripheral blood. These results suggested that AHCC may display a protective role against opportunistic fungal infection in leukopenic hosts.