Summary Two polyoma tumors in mice have been found to have a 1,000-fold increased transplantability if the IC or IT routes of inoculation are used instead of SC. When polyoma virus-immune mice were challenged by the IC or IT routes, the degree of demonstrable resistance was 105 and 104 minimal transplantable numbers of cells as compared to 101 by the SC route. Mice immunized IC or SC with a single dose of X-irradiated tumor cells and later challenged IC showed protection in the IC but not the SC immunized group. The ability of in vitro polyoma transformed tissue culture cells to produce tumors on inoculation of isologous adult mice was most markedly enhanced by whole body X-irradiation, but best results were obtained if IC inoculation was made into irradiated animals.