Intraperitoneal administration of lead acetate, lead carbonate, lead chloride, lead nitrate or lead oxide at 0.5 or 6 mg per kg per day on five consecutive days was found to produce diverging effects on delayed hypersensitivity to sheep erythrocytes in Balb/c mice according to the salt used. Lead carbonate, lead nitrate and lead oxide exerted immunosuppressing properties, while lead acetate and lead chloride enhanced this cell-mediated immune response. From these findings, it is concluded that lead immunotoxicity critically depends on which salt is present.