Cell-free ascitic fluids obtained from mice bearing Ehrlich ascites tumors contain an inhibitor which prevents vaccinia virus hemagglutination. Serial passage of the virus in tissue cultures of L cells in the presence of the inhibitor favors the appearance of virus which has lost the ability to produce hemagglutinin. Serial passage of the virus under comparable conditions in the absence of the inhibitor did not influence hemagglutinin-producing activity of the virus. These results support the assumption that the loss of hemagglutinin-producing ability by baccinia virus, on passage through the Ehrlich tumor, results from the action of the inhibitor.