The prophylactic effects of a Basidiomycetes preparation, AHCC, against experimental opportunistic infections were investigated in leukopenic mice. In cyclophosphamide-induced leukopenic mice, oral or intraperitoneal administration of the AHCC at doses of 1000 or 50 mg/kg/day, respectively, for 4 consecutive days prior to Candida albicans infection significantly prolonged the survival periods of the infected mice, and decreased the viable counts of C.albicans cells recovered from their kidneys. Similarly, the oral treatment with AHCC protected mice from lethal infection with Pseudomonas aeruginosa and intraperitoneal one also protected mice from infection with methicillin-resistant Staphylococcus aureus (MRSA). These results suggest a potential usefulness of the AHCC as a propylactic agent for the management of patients with opportunistic infections.