Effects of a dietary supplement, d-Lenolate, on the host defence in immuno-compromised mice were investigated in terms of the restoration of leukocytopenia and the protection against several microbial infections. Lenolatepotentiated phagocytic activity of rat or mouse peritoneal yolymorphonuclear leukocytes (PMN) in vitro or in vivo, respectively. The agent enhanced nitroblue-tetrazoliiim (NET) reduc- ing capacities of rat and guinea pig polymorphonuclear leukocytes in the peripheral blood. Oral or intraperitoneal administration of Lenolate into mice prior to X-irradiation or treatment with cyclophosphamide (CY) prevented the leukocytopenia to some extent and promoted the restoration in the pfwgocytic activity of both the peripheral blood leukocytes and peritoneal macrophages. In addition, IL-1 production by mouse macrophages was augmented by cultiiring with the agent in vitro. Oral administration of Lenolate increased significantly the survival rates of X-irradiated mice against acute systemic infections with Escherichia coli, Pseudomonas aeruginosa and Candida albicans, and intramuscular infection with Esherichia coli.