Laboratory and field studies have demonstrated that the immune system is sensitive to environmental contaminants. Testing protocols have been developed to screen for immunotoxic effects and elucidate mechanisms of toxicity in laboratory rodents. Similar methods have been applied to wildlife species in captivity and the wild. Several epizootics in wildlife have been associated with elevated exposure to contaminants. This paper discusses immunotoxicological techniques used in studies of avian wildlife. Measurements of immunological structure include peripheral white blood cell counts and the mass and cellularity of immune organs such as the thymus, spleen, and bursa of Fabricius. While contaminants can alter these measures of immunological structure, such measures do not directly assess how the immune system functions, i.e., responds to specific challenges. The two most commonly used in vivo immune function tests in birds are the phytohemagglutinin (PHA) skin response for T cell-mediated immunity and the sheep red blood cell (SRBC) hemagglutination assay for antibody-mediated immunity. In vitro tests of immune function in avian wildlife include proliferation of lymphocytes in response to various mitogens and phagocytosis of fluorescent particles by monocytes. While optimization of in vitro techniques for wildlife species is often time-consuming, these assays usually require only a single blood sample and can elucidate mechanisms of toxicity. In immunological studies of wildlife, investigators should consider factors that may influence immune responses, including age, body condition, date, developmental stage of the immune system, and time required for the progression of immune responses.