Two independent transgenic mouse lines carrying human hybrid c-Ha-ras genes with their own promoter region encoding prototype products, were established. In these lines, about 50% of transgenic offspring had tumors within 18 months. The tumors developed in restricted tissues and about 60% of affected mice had angiosarcomas. The transgenes were expressed both in the tumors and in all normal tissues. However, somatic mutational activation was detected only in the transgenes of the tumors. The point mutation at the 61st codon, from CAG(Gln) to CTG(Leu), was detected in all angiosarcomas (22/22), some lung adenocarcinomas (3/11) and Harderian gland adenocarcinomas (4/7) in both lines. The other point mutation at the 12th codon from GGC(Gly) to GTC(Val) was detected in two of the four skin papillomas. No mutations on these codons were detected in normal tissues of transgenic mice. Nontransgenic littermates had no tumors at all. From these results, it was strongly suggested that the mouse tumors do not develop only by the expression of the transgenes, and that definite somatic point mutation of the human c-Ha-ras transgenes in certain cell types may be a causative event in tumorigenesis in these transgenic mice.