Tumors are characterized by their ability to avoid the host immune system. Ehrlich ascites tumor cells were used to investigate the early alterations of the host immune system after tumor inoculation. The results show that frequencies of splenic Th lymphocytes were drastically reduced during tumor growth, reaching a minimum only two days after tumor inoculation. The frequency of splenic CD4⁺ lymphocytes expressing IFN-γ was significantly increased, although the total number was unchanged, suggesting that there was no net induction of Th1-type response. Splenic macrophages were increased, in both frequency and cell number, after four days of tumor growth. The same pattern was observed when mice were inoculated with cell free ascitic fluid. TGF-β precursors were detected in tumor cells as well as in ascitic fluid. The data suggest that tumor actively interacts with host immune system by means of tumor cell secreted factors.