Development of hard clam (Mercenaria mercenaria) larvae was examined as a potential biomarker of exposure to polychlorinated biphenyls (PCBs). Clams from clean sites in Massachusetts Bay were thermally induced to spawn in the laboratory using seawater, and the gametes were collected and pooled by sex prior to fertilization. The larvae were cultured in seawater amended with Aroclor 1254, at concentration ranges bracketing environmentally relevant levels. A dose-response relationship was observed for larval development in the presence of Aroclor 1254; at higher doses, fewer larvae developed to the normal straight-hinge, or D-shaped stage, relative to the controls, while the number of abnormally shaped larvae increased.