Hyphae and oospores of Lagenidium giganteum Couch were tested for toxicity, infectivity, and irritancy to mice, rats, and rabbits. No infection occurred following exposure to the entomopathogen. Intratracheal instillation of both viable and autoclaved oospores produced a moderate, focal, granulomatous bronchopneumonia, and intraperitoneal injection of viable and autoclaved hyphae produced multiple abscesses in the liver, spleen, and mesentery. Dermal exposure of rats and ocular exposure of rabbits to L. giganteum did not produce significant irritation. Lesions seem to be the result of body reactions to large quantities of foreign material. We conclude that it is unlikely that L. giganteum is infectious or irritating to man or animals.