Ehrlich ascites tumor grows in almost any mouse host. We have found a strain of dystrophic mice to be strongly resistant to Ehrlich ascites tumor. Mice of the dystrophic strain survived injection of Ehrlich Ascites tumor cells that had just been passaged through either dystrophic mice or C57BL X DBA/2 F1, (hereafter known as BD2F1) mice. However, most of the dystrophic mice died when they were given injections of Ehrlich ascites tumor cells that had just been passaged through Swiss white mice. All Swiss white and BD2F1 mice died when they were inoculated with Ehrlich ascites tumor cells, regardless of the type of mouse through which the Ehrlich ascites tumor cells had just been passaged. In some dystrophic mice, the tumor grew to a palpabale size and then disappeared. This provided the opportunity to passage the tumor from one dystrophic mouse to another and thereby to demonstrate resistance in the dystrophic mice to tumor that had grown previously in dystrophic mice. There was no detectable difference in the rate of tumor growth in either Swiss white or BD2F1 mice.