Fish exposed to toxic pollutants both in the wild and in the laboratory show significant changes in the immune activity of kidney macrophages. Decreases in the chemotactic and phagocytic responses and increases in neutral red uptake and melanin accumulation were observed in several species of fish captured in the Elizabeth River, Virginia, which is highly contaminated with polynuclear aromatic hydrocarbons (PAHs). The chemiluminescent response of macrophages was inhibited by laboratory exposure of fish to toxic sediments and by in vitro exposure of macrophages to tributyltin (TBT). Results of these assay techniques are presented with the aim of providing reliable bioindicators of the effects of environmental contamination on fish health.