RNA expressions of common integration site (int) genes and several cnogenes were investigated in mammary carcinomas spontaneously developed in different three strains of mice ; DD/Tbr, NIH Swiss and BALB/c which harbor DD-MMTV derived from DD/Tbr mouse. Latter two strains of mice were designated NIH/Mtv⁺and BALB/Mtv⁺, respectively. An increased expession ofint-1 (wnt-1) andint-2 genes was observed in 56% (9/16) and 50% (8/16) of mammary carcinomas of DD/Tbr mice, respectively. Eitherint-1 orint-2RNAs were expressed in 81% (13/16) of the carcinomas of DD/Tbr mice. IN NIH/Mtv⁺mice, activation ofint-1 andint-2 was observed in 41% (7/17) and 24% (4/17) of mammary carcinomas, respectively. Eitherint-1 orint-2 RNAs were expressed in 47% (8/17) of the carcinomas examined in this strain. In BALB/Mtv⁺mice, on the other hand, eitherint-1 orint-2 genes were transcribed into RNAs at low frequency (33% : 3/9) . These results suggest that the frequency of activation ofintgenes in mammary carcinomas induced by the same DD-MMTV in three strains of mice is genetically defined characteristics of these strains, and that the involvement ofint-1 andint-2 genes in virus-induced mammary carcinogenesis may be influenced by genetic properties of animals. The activation ofint-1 andint-2 genes did not clearly correlate with an increase in the expression of oncogenes examined ; H-ras, K-ras, N-ras, myc, raf, fgr, fms, erbB, mos, andsrcgenes.