We now keep HFRSV free WF-Osaka rats and ACI rats together in the separate three animal rooms (animal room 1, 2 and 3) and the incidence of colon carcinoma is still high on the WF-Osaka rats in animal room 1 with the high humidity. Two female ACI rats developed colon carcinomas in the ascending colon. The gross and the histological appearance of the colon carcinoma were completely the same as those of WF-Osaka rats. ACI and WF-Osaka rat strain together have been kept bred in neighborhood of each other in different racks in the identical animal room 1. To obtain HFRSV free ACI rat strain, Antecedents born by cesarean section of ACI female pregnant rat were foster-bred by WF-Osaka female nursing rat incidentally, and at the fourth mating generation after the start of foster-breeding, they developed colon carcinomas at the age of four months. Before five out of eight F1 hybrids by WF-Osaka cancer carrying female rat x male ACI rat had developed the same colon carcinoma, but none of F1 hybrids by the contrary mating had developed colon carcinomas in this same animal room 1. Animal room 1 and 2 where there was a high incidence of colon carcinomas, had happened to be kept moistened. However, after disinfection of these animal rooms, the animal room 2 and 3 occurred to be kept dried, and rats of WF-Osaka strain ceased to develop colon carcinomas in the animal room 2. Thereafter, animal room 2 and 3 were adjusted to be kept moistened again. Subsequently WF-Osaka rats in the animal room 2 began to have colon carcinomas in the ascending colon as before, but none of rats developed colon carcinomas in the animal room 3. Based on these findings, we consider that milk factor at the time of foster-breeding played an important role first and high moistened condition of the animal room resulted in promoting effect on colon carcinogenesis on ACI rats and WF-Osaka rats as well.