Because fish respond to bacterial pathogens by producing serum antibodies, it was possible to construct a seasonal serological record in three species from clean and polluted waters of the New York Bight. Antibody levels were determined by testing sera for agglutinating activity against 36 characterized and 17 uncharacterized strains of bacteria. During warm months, summer flounder from the polluted area had significantly higher antibody levels and antibodies to a greater diversity of bacteria than fish from the unpolluted area. Weakfish from the same polluted area shared with summer flounder raised titers to many bacteria. The greatest proportion of raised titers was against Vibrio species; although prominent titers were also seen against Aeromonas salmonicida and Hemophilus piscium, bacteria usually associated with diseases in fresh water but not marine fish. Differences between polluted and clean waters were not as evident in winter flounder during cold months. No convincing evidence was found for a bacterial cause of fin erosion disease. Data illustrate the usefulness of the serum antibody record in identifying environmental exposure to bacteria in marine fish and indicate that the polluted New York Bight apex has increased levels and diversity of bacteria during warm months.