To investigate whether the presence of an activated ras oncogene influences the ability of tumour cells to metastasize, the c-Ha-ras-1 oncogene cloned from EJ/T24 cells was introduced into MT1 Cl.5/7 mouse mammary carcinoma cells. Since the MT1 Cl.5/7 cells are already tumorigenic but have a low metastatic capacity, this experimental design allows a distinction to be made between the effects of the ras gene on metastasis and tumorigenicity. MT1 Cl.5/7 containing the EJ c-Ha-ras-1 metastasized more readily and to more tissue sites than control cells (2.8 sites/mouse vs 0.9 sites/mouse). The metastases expressed the EJ c-Ha-ras-1 p21 ras proteins; however, one metastasis was discovered that had lost the expression of the c-Ha-ras-1 gene. When these cells were re-tested for metastasis, the rate of metastasis was indistinguishable from that of controls. This observation, coupled with a demonstration that lung colonization potential following intravenous inoculation is unaffected by the presence of the activated ras gene, argues that the effect of mutant ras genes is exerted on the ability of cells to escape from the primary tumour, rather than on a survival in the circulatory systems and ability to seed a second site.