The spontaneous transformation of cultured mouse embryonic fibroblasts (MEF) from the outbred CD-1 mouse strain was investigated. Four MEF clones, obtained from four different mouse embryos, were cultured continuously for periods ranging from 300 to 400 days. MEF cells entered a "crisis" with acute loss of proliferation capacity after 16 +/- 2 days of culture. Surviving cells gradually acquired properties of transformed cells with the following times: the capacity to grow at a cell density of 10(5) cells/cm2 was acquired after 269 +/- 45 days. Twenty-five % anchorage-dependent colony forming ability was observed after 255 +/- 39 days. 0.5% anchorage-independent colony forming ability was observed after 290 +/- 54 days. Protective factors capable of significantly delaying the above events might be investigated with the MEF system.