Shionogi Carcinoma 115 (SC 115) is an androgen-dependent mouse tumor, and Chiba subline 2 (CS 2) is its androgen-independent subline which differs from SC 115 in cell size, amount of androgen receptors, and karyotype. To shed light on the mechanism of clonal selection of androgen-independent tumors, mixed tumors with SC 115 and CS 2 were prepared, and growth of these tumors was examined in vivo and in vitro. When the mixed tumor was transplanted in mice, CS 2 showed a predominant growth over SC 115. In a culture of mixed tumor cells, however, CS 2 showed no selective growth advantage. The suppressive interaction which occurred in vivo was due neither to transferable substances, nor to some immunological factor(s). It may be, at least partially, attributable to necrosis formation in SC 115, which developed with an increase in the size of the tumor.

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Supported in part by a Grant-in-Aid for Cancer Research from the Ministry of Education, Science, and Culture (62010012); from the Ministry of Health and Welfare (62-40); and for scientific research from the Ministry of Education, Science, and Culture (62480333), Japan.

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