SV40 T-antigen-expressing human cells generally have an extension of lifespan until a period called "crisis" begins. On rare occasions a clone of cells emerges from the population in crisis and gives rise to an immortalized cell line. The present study compares the frequency of immortalization of cells from two different human lineages, lung fibroblasts and mammary epithelial cells. Most of the T-antigen-transfected clones from both cell types failed to immortalize, however, within those clones which were immortalization-competent the frequency of escape from crisis was found to be much higher (10(-5)) in mammary epithelial cells than in human fibroblasts (3 x 10(-7)). The frequency of escape from crisis in fibroblasts could be increased by chemical mutagenesis or by infection with a defective retrovirus. T-antigen-transfected fibroblasts were uniformly highly aneuploid both before and after crisis. In contrast, many SV40 T-antigen- and human papilloma virus 16 E6- or E6/E7-transfected mammary epithelial clones maintained a subpopulation of pseudodiploid cells prior to crisis, and the immortal cells that emerged following crisis were generally pseudodiploid. The higher frequency of escape from crisis in mammary epithelial cells is best explained by postulating a mutational inactivation of one allele of a critical gene followed by the elimination of the remaining wild-type alleles, with a much higher frequency of this second event in mammary epithelial cells due to their reduced ploidy compared to that in T-antigen-transfected fibroblasts. The results are discussed in terms of the regulation of telomerase activity and the M1/M2 model of cellular senescence.