1. Genotoxicity experiments were conducted with cultured fish cells to determine if the high frequency of epidermal papillomas observed in lemon sole from Sturgeon Bank, where a sewage treatment plant discharges, could be correlated with contamination of the sediments with chemicals such as 3,4-benzopyrene. 2. The frequency of chromosome aberrations was measured in cultured Umbra limi heart (U1-H) and Chinese hamster ovary (CHO) cells following exposure to the polycyclic aromatic hydrocarbons (PAH) 3,4-benzopyrene (BP), 1,2,5,6-dibenzanthracene (DBA), 1,2-benzanthracene (BA), and pyrene (PY), activated using S9 prepared from rainbow trout liver. 3. An increase in the chromosome aberration frequency was only observed following exposure to fish S9-activated BP in both cell lines. 4. Following exposure of the cells to both Sturgeon Bank and Spanish Bank sediment extracts, it was determined that a higher level of toxic and genotoxic activity was associated with the Sturgeon Bank sediments. 5. Since the detection of PAH genotoxicity requires the presence of S9, and since a higher level of genotoxic activity was noted following sediment extract exposures with no S9 present, this suggests that the extracts contain a complex mix of chemicals, some of which express genotoxic activity. 6. An assessment using the micronucleus test failed to indicate in vivo genotoxicity in fish collected from Sturgeon and Spanish Banks. 7. It was, therefore, difficult to associate the observed sediment genotoxicity with the previously noted high incidence of epidermal papillomas in lemon sole from this area.