In vitro invasion and in vivo metastasis assays were performed with a panel of MCF-7 cells transfected with isogenic constructs of mutated rasH genes. Both increased levels of rasH expression and rasH oncogene activation increased activity of derivative cell lines in in vitro invasion assays. In vivo formation of spontaneous metastases was assessed after intradermal inoculation of MCF-7 cells in the vicinity of the mammary fat pads of ovariectomized nude mice. No metastases were seen in the absence of estradiol treatment of the mice. With estradiol supplementation of the mice both the rasH-transfected and control transfected cell lines gave a higher incidence of metastases than parental MCF-7 cells. Prolonged treatment of mice with exogenous estradiol (60 days vs. 21 days) resulted in more frequent metastases to liver and lung at the end of the 90-day observation period. In contrast to activated rasH-gene enhancement of metastatic capacity of rodent fibroblast and epithelial cell lines, there was no correlation of rasH expression with in vivo metastatic capacity of a human mammary carcinoma cell line.