Among spontaneously occurring tumors, pulmonary tumor, malignant lymphoma, mammary tumor, and ovarian tumor were the major ones in 232 ddY female mice. The former three tumors appeared significantly earlier and the latter one increased in incidence in 229 mice given 600 R whole or 800 R partial body (tunk) x-irradiation at the age of 10 weeks. Five tumors were examined electronmicroscopically from each tumor type of both the spontaneously occurring and radiation-induced tumors. C type virus particles were present only in the malignant lymphoma and B particles in the mammary, ovarian and pulmonary tumors, which are all of epithelial character. Thus, a s far as we were concerned in this mouse strain, B particles were produced preferentially in epithelial cells and C particles in non-epithelial cells. The retrovirus particles were found in the same frequency, namely, 10 of 20 tumors examined in both the tumor groups. From our results, the intervention of virus in radiation carcinogenesis still remains in question. ACTA PATHOL. JPN. 31: 233-247, 1981. and several other mammal species.24 The retrovirus has been classified into A, B, and C types according to the morphological criteria (BERNHARD, 1960)2. The C particles have been frequently found in mesenchymal tumors such as lymphoma including and other sarcomaslO and the B particles in mammary tumorslB and less in non-mammary tumors arising from the l~n g ,~, l * testis,l8 ~~a r y~~~~ and so on. As yet,