Environmental chemicals, including pesticides, have the potential to alter the immune response of laboratory or free-ranging animals. As a consequence, wild animals may become more susceptible to microbial or parasitic diseases; there is ample evidence that free-ranging wildlife frequently are diseased. Mathematical models predict that disease and parasitism could alter host population growth rates and that host susceptibility is an important component of the growth rate function. Laboratory studies have shown that all classes of insecticides and fungicides have immunosuppressive properties. Only a few studies have been done with wildlife species (fish, mallards, wild mice, and earthworms). A battery of immune function tests that has been developed recently for captive birds is now being used in free-ranging birds to determine if pesticide use or chemical disposal may be at least partially responsible for recent increases in the incidence of disease outbreaks in free-ranging wildlife.