A mammalian in vitro system was used to evaluate the genotoxic potential of two complex environmental samples. Sister chromatid exchanges (SCEs) were measured in Chinese hamster V79 lung fibroblast cells, following exposure to whole extracts of sediments collected from a highly contaminated harbor--Black Rock Harbor, Connecticut (BRH)--and from a reference site--central Long Island Sound (CLIS). Characterization of BRH sediment by analysis of prepared chemical extracts revealed high concentrations of polyaromatic hydrocarbons, anthraquinones, carbazoles, and several inorganic substances that are genotoxic and capable of inducing SCEs. The CLIS sediment, although cleaner than BRH sediment, does contain low levels of similar contaminants. For instance, BRH sediment contains 3.16 micrograms g/g of the polynuclear aromatic hydrocarbon benzo(a)pyrene (BP), whereas CLIS sediment contains only 0.807 micrograms g/g BP. Sediments were extracted with organic solvents and redissolved in dimethyl sulfoxide (DMSO) for culture exposure. Results indicated a concentration-dependent increase in SCEs in cells exposed to whole BRH sediment extracts and a smaller but significant increase in SCEs in cells exposed to whole CLIS sediment extracts. Four times as much CLIS material (0.13 g extracted sediment (dry wt.)/ml of exposure media) was needed to induce a significant doubling in SCEs, compared to BRH material, in which only 0.03 g extracted sediment (dry wt.)/ml of exposure media was needed.