Mugil so iuy was exposed to different concentrations of benzo(a)pyrene, pyrene and their mixture of equal quantity, and its haemoglobin contents were examined. The results showed an obvious time response and dose effect. Under the same concentrations of 0.1,1.0,5.0,10.0 and 50.0 μg/L, the mixture of the two pollutants showed the greatest toxicity to haemoglobin content, while pyrene the lowest. The joint action of benzo(a)pyrene and pyrene was synergism. Fig 1, Tab 1, Ref 5