The effect of pentachlorophenol (PCP) on reactive oxygen intermediate (RO1)-mediated bactericidal activity was studied in an estuarine teleost, Fundulus heteroclitus. Fish were exposed in vivo for 12 d to 50 pg 1-' (nominal) PCP, an environmentally relevant, sublethal concentration. Fish were then sacrificed, and macrophages and eosinophils isolated from the pronephros. Cells were assayed for their ability to phagocytose FITC-conjugated yeast, and produce the ROls superoxide (02-) and hydrogen peroxide (H202). Cells were also assayed for luminol-enhanced chemiluminescence (LCL), a measure of activity of the H202-peroxidase-halide bactericidal system. Finally, cells were tested for bactericidal activity against Listonella anguillarum, a marine bacterium responsible for vibriosis in fish. In unexposed fish, eos~nophils were more active than macrophages in defense related mechanisms. The 0; production, LCL activity, and bactericidal activity of phagocytosing eosinophils were significantly greater than macrophages; higher levels of phagocytosis and H 2 0 2 production by eosinophils were also observed. Significant reductions in eosinophil phagocytic activity were produced by PCP exposure. Phagocytes from PCP-treated fish showed decreased bactericidal activity, which was statistically sign~f~cant in the case of macrophages. Both phagocytic cell types had significantly increased basal levels of 0 2 -and LCL after PCP treatment. However, PCP had no effect on the phagocytically stimulated levels of these activities, nor on the stimulated production of H202. Modulation of immune activities of these phagocytic cells by xenobiotics, as seen In these experiments, has the potential to alter susceptibility to disease causing agents such as Listonella.