It is well known that in chemically induced sarcoma (rats) or epithelioma (mice) tumor-specific antigens are present, which differ from the tissue-specific antigens normally present in the tissues from which these cancers arise. However, the host tolerates the tumor and is apparently unable to elaborate antibodies active enough to reject the tumor. On the other hand, it is well known that the specific-tumor antigens elaborated by different cancers, though arising from a same tissue and produced by a same carcinogen, are often chemically different. These chemical differences between tissue-specific antigens constitute a major difficulty to use them to induce an immunoprophylaxis against the induction of cancer of the same tissue with a same carcinogen. Nevertheless, we have previously demonstrated that by using the specific antigens, present in a pool of microsomes extracted from cancer cells, it is possible to induce a marked degree of resistance towards the induction of a similar type of cancers in the immunised animal. In this paper it is shown that, by chemical procedure, it is possible to extract, from a pool of nuclei of cancer cells, two fractions which can induce a highly significant degree of resistance in the injected animals towards the incidence of cancer of a similar type. The two types of cancers investigated were the subcutaneous sarcoma of the rat induced by injection of methylcholanthrene and the skin cancer of the mouse induced by cutaneous methylcholanthrene paintings.