In spite of extensive laboratory data on the effects of chemicals and drugs on immunologic parameters in laboratory animals, and a well established correlation between suppression of immune function and increased incidence and/or severity of certain infectious and neoplastic diseases, interpreting data from experimental immunotoxicology studies for risk assessment purposes has proved challenging. This is particularly true when the immunological effects are minimal-to-moderate in nature, as might be expected from inadvertent chemical exposures. This review examines the methods used to evaluate immune responses in laboratory rodents and their utility to predict disease outcomes. The available data suggest that if a large enough population is exposed and that the challenge dose or virulence of pathogenic organisms or tumor cells is sufficient, small changes in immune surveillance could increase the background incidence and burden of disease in the human population.