Polycyclic aromatic hydrocarbons (PAHs) are an important class of environmental pollutants that are known to be carcinogenic and immunotoxic. Many authors have focused on macrophage activities in fish exposed to PAHs. However, fewer studies have reported decrease in specific immunity in such fish. We investigated the intracellular mechanisms by which the 3-methylcholanthrene (3-MC) decreased lymphocyte proliferation in carp. T- and B-lymphocyte proliferation induced by Concanavalin A (Con A) and lipopolysaccharide (LPS) were inhibited by 3-MC (0.5-50 microM). 3-MC also produced a rapid and a sustained increase in intracellular calcium concentration ([Ca(2+)](i)) (2 h minimum). However, the cytochrome p450 1A and Ah receptor inhibitor, alpha-naphtoflavone (a-NF), also inhibited lymphocyte proliferation and did not reverse the effects of 3-MC. Moreover, since a-NF and 3-MC increased [Ca(2+)](i) and inhibited lymphocyte proliferation it was possible that calcium release played a role in 3-MC-inhibited lymphocyte proliferation. The rise in [Ca(2+)](i) induced by 3-MC was potentiated by the inhibitor of the endoplasmic reticulum calcium ATPases, thapsigargin. Treating cells with 3-MC decreased calcium mobilization caused by thapsigargin. These results suggest that 3-MC acts on the endoplasmic reticulum, perhaps directly on calcium ATPases, to increase intracellular calcium levels in carp leucocytes.