Waste motor oil concentrations of 20 ppm and higher induced lesions in the vascular systems of Atlantic silversides and oyster. These lesions were associated with the pseudobranch, the heart, and the arterial system of the Atlantic silversides, and the branchial efferent vein of the oyster. Lesions occurred in the gastro-intestinal tract of oyster, the gill and kidney of scallop, and the mantle of both species. Based on these preliminary exposures, the scallop, of the three species tested, demonstrated the highest sensitivity to the toxic effects of 20 ppm waste motor oil, or higher, for periods up to 60 days. The LC-50 (96 hr) value for waste motor oil and adult Atlantic silversides was 1,7000 ppm. Acute toxicity values were not determined for the oyster or scallop.