The authors studied the combined carcinogenic activities of MCYST and organic pollutants in water of Dianshan Lake, using in vitro Syrian hamster embryo (SHE) cell transformation and immunohistochemical assays. The results showed that MCYST alone could not induce SHE cell transformation, but it increased the effects of low dose organic pollutants in cell transformation in a dose-dependent manner. In the cells isolated from type II transformed foci, ras P21 protein showed a positive immunohistochemical staining. It suggests MCYST may be a tumor promoter, which has a synergistic carcinogenic activity organic pollutants in water and the activation of ras oncogene may be one of the factors inducing malignant transformation.