Carcinogenic effect of N, N'-dimethylnitrosourea (DMNU)
on mice and hamsters was studied. Repeated subcutaneous injections　of DMNU resulted in the induction of malignant lymphomas with an　incidence of 100 per cent in adult C3HjBifBjKi mice and induced　malignant tumors of　forestomach, mammary gland and uterus with a　high incidence in adult hamsters. Control animals showed no pathological　changes. Electron microscopy revealed the presence of murine　type C virus particles in some of the tissues examined. Many type C　virus particles were found in a transplant of DMNU.induced malig.　nant lymphoma. Some type C virus particles were shown in malignant　lymphomas and lymph nodes of malignant lymphoma-bearing mice.　A very small number of type C virus particles were observed in thymus　of control mice and bone marrow of a malignant lymphoma.bearing　mouse. A few particles, quite similar to murine type C virus particles,　were detected in　DMNU.induced mammary adenocarcinoma of hamster.　No virus-like particles were seen in mammary glands of control
hamsters. Whether these particles are merely passengers or are playing　a significant role in the carcinogenesis of these tumors remains to　be determined.</p
