The subtle effects that low levels of pollutants have on fish populations are probably more important than the effects of large spills, since the effects are less likely to be obvious and the source more difficult to detect in time to save the environment. An experiment was carried out to determine if exposure of striped bass to sublethal concentrations of chlorine affected their susceptibility to bacterial infection. Exposure of striped bass for 96 h to sublethal concentrations of total residual chlorine (TRC) (0.05-0.23 mg/L) did not increase their susceptibility to infection with the bacterial pathogen Vibrio anguillarum. Variables examined were TRC concentrations, length of exposure to chlorine, and the order of exposure to chlorine and the pathogen. Mortalities in the groups exposed to both chlorine and pathogen were not significantly different from those seen in groups receiving the bacteria only. Smaller fish are more susceptible than larger fish, and the LD50 is markedly affected by the ambient temperature in that fewer bacteria are needed to kill fish at lower temperatures. One contributing factor to this increased resistance of fish at higher water temperatures appears to be related to their immune status. 29 references, 5 tables.