Four vasoformative sarcoma in vivo tumor lines arising from the s.c. implantation of 3 .times. 104 BALB/3T3 [fibroblast] cells attached to a 1- .times. 5- .times. 10-mm polycarbonate platelet were similar in chromosome constitution to the parental 3T3 line. All tumors contained the same marker chromosomes that were present in the parent BALB/3T3 [mouse] cells. The findings conclusively showed that the tumors derived from BALB/3T3 cells and not host cells or in vivo fusion of inoculated 3T3 cells with host cells. Each tumor line had a unique karyotype that was more homogeneous than that of parental BALB/3T3 cells, suggesting that each tumor represented a separate clone. An M1 marker chromosome was consistently present in each of the 4 tumor lines but was present in only about 50% of the parental BALB/3T3 cells. It appeared to be a distinct and stable feature of the tumors.