Tumor cells suppress host immunity by secreting soluble factors, among which cytokines play important roles. Type I cytokine mRNA levels (IL-2 and IFNgamma) and type II cytokine mRNA levels (IL-4, IL-10 and IL-13) of twenty tumor cell lines and fresh tumor tissues from fifteen patients with colorectal cancer were examined by RT-PCR method. Upon analysis, tumor cells showed type II predominance and were resistant to NK cytolysis. In order to reverse the type II predominant status, type I cytokines (IL-2 or IFNgamma) and/or anti-type II cytokine antibody (IL-10 mAb) were used in an in vitro study. By this shifting treatment, type II cytokine mRNA levels of these tumor cells were decreased, moreover, the sensitivity to natural cytolysis of these cells was then significantly increased. Thus, tumor-induced type II cytokine dominance is possibly to be shifted into type I status, which benefits the anti-tumor natural immunity.