Tunicates are filter-feeding estuarine and marine animals that are frequently exposed to chronic environmental pollution. This study demonstrates that exposure to low-level (i.e., below the threshold of acute lethality) contamination with tributyltin, creosote, and copper can have substantial effects on natural immune reactions in tunicares. Sublethal doses of toxicants administered either in vitro or in vivo profoundly affected phagocytosis, cellular cytotoxicity, and hematopoietic cell proliferation. Effects were not always inhibitory, and responses often varied depending on the route of toxicant administration. The data suggest that pollutants can activate cascades of cellular processes and compensatory mechanisms, as well as directly inhibiting some of the responses tested. Some evidence indicates that toxicants exert their effects by altering the relative frequencies of circulatory hemocytes.