The AP-1 transcription factors are considered immediate-early response genes and are thought to be involved in a wide range of transcriptional regulatory processes linked to cellular proliferation and differentiation. To study one of the key members of this family, the proto-oncogene c-jun, we have used homologous recombination-mediated gene targeting to produce mice with a c-jun null mutation. c-jun null embryos die at mid-gestation, with an average time of death of 12.5 days postcoitus. Homozygous mutant embryos are indistinguishable from wild-type littermates both grossly and histologically until the time of death. However, primary fibroblasts derived from live heterozygous and homozygous mutant embryos show greatly reduced growth rates in culture. The subnormal mitogenic response of these cells cannot be overcome by the addition of a number of purified mitogens. These studies indicate that although c-jun is not required for cellular proliferation and differentiation up to mid-gestation, it is required for survival past that stage as well as for the mitogenic response of embryonic fibroblasts in culture.