Fibroblast cultures were derived from mouse embryos containing either one (p53+/-) or two (p53-/-) inactivated p53 alleles and compared to normal embryo fibroblasts for a number of growth parameters. Early passage p53-deficient embryo fibroblasts (p53-/-) divided faster than normal embryo fibroblasts, achieved higher confluent densities, and had a higher fraction of division-competent cells under conditions of low cell density. Flow cytometry studies of early passage embryo fibroblasts showed that the percent of p53-deficient cells in G0/G1 was lower than in normal cells, consistent with the argument that p53 mediates a G1 block. When p53-deficient and normal cells were passaged for long periods of time, the homozygote (p53-/-) fibroblasts grew at a high rate for over 50 passages and never entered a non-growing senescent phase characteristic of the heterozygote (p53+/-) and normal (p53+/+) cells. The p53-deficient fibroblasts were genetically unstable during passaging, with the p53-/- cells showing a high degree of aneuploidy and the p53+/- cells displaying a moderate level of chromosomal abnormalities by passage 25. Surprisingly, the heterozygote cells lost their single wild type allele very early during culturing and in spite of this loss most heterozygote lines entered into senescence. We conclude that the loss of p53 by itself is insufficient to confer immortality on a cell, but does confer a growth advantage. Taken together, the findings confirm that the absence of p53 promotes genomic instability, which in turn may result in genetic alterations which directly produce immortality.