Primary murine Rous sarcoma was produced in adult mice of seven strains, C57BL/6, DBA/2, BALB/c, C3H/He, CBAJ, AKR, and DDD, by s.c. inoculation of a mixture of 5 X 10(6) chicken tumor cells containing Schmidt-Ruppin Rous sarcoma virus and 9- to 12-day-old mouse embryo cells (MEC) (2 X 10(6) ) of the syngeneic strain. The sarcoma developed at the site of injection in almost all mice tested, but there were some differences in the latent period and the survival time among mouse strains. When the

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... umber of cells inoculated was reduced to 5 X 10(4) for chicken tumor cells induced by the Schmidt-Ruppin strain of Rous sarcoma virus (SR-CTC) and 2 X 10(4) for MEC, no tumor was produced in C3H/He mice. These tumors had strain specificity and the Schmidt-Ruppin strain of Rous sarcoma virus genome in masked form. The tumor at the site of injection originated in the embryo cells injected along with SR-CTC. This was confirmed by CBAT6/T6 marker chromosome analysis of the tumor cells of CBA mice induced with SR-CTC plus CBAT6/T6 MEC and also confirmed by transplantation of a C57BL/6 X C3H/He F1 tumor which had been induced with SR-CTC plus C3H/He or C57BL/6 MEC. Tumor induction in adult mouse by a mixture of virus and syngeneic 9- to 14-day-old embryo cells was tested for human adenovirus serotype 12 (Ad12) and simian virus 40. Primary Ad12 tumor was also induced in adult CBA, C3H/He, and DDD mice by 4 X 10(5 to 6) 50% tissue culture infective dose of Ad12 with 5 X 10(6) syngeneic embryo cells. This tumor contained Ad12 T-antigen-positive particles in cells. But in the case of simian virus 40, the tumor did not appear for about 300 days of observation.