Tilapia were exposed by intraperitoneal injection to the polycyclic aromatic hydrocarbon (PAH), benzo[a]pyrene (B[a]P). Histopathologic evaluation of the fish primary hematopoietic compartment, the pronephros, demonstrated increased vacuolation of both stromal and parenchymal cells, as well as dramatic reduction of lymphoid elements. Total pronephros cell counts were also diminished in a dose-dependent manner by the chemical exposure. The oxidative metabolic burst in phorbol myristate acetate (PMA) stimulated macrophages isolated from the pronephros was significantly inhibited by B[a]P, but only at the highest dose level employed. The phagocytic capacity of pronephros macrophages was not altered by the chemical treatment at any experimental dose level. These indicate immune cell alterations may occur in the fish pronephros at doses of B[a]P not producing overt toxicity. However, altered pronephros macrophage activity in treated fish was either not observed or was seen in the presence of other chemical-induced toxic responses, including decreased activity and reduced food intake.