The mechanism by which SV40 large T-antigen immortalizes human lung fibroblasts is not yet understood, and the frequency with which immortalization occurs is unknown. Here we report detailed studies of the kinetics of immortalization. Approximately 20-50% of individual T-antigen transfected clones of IMR-90 human lung fibroblasts are able to immortalize. The failure of some clones to immortalize is consistent with the hypothesis that in some cells the aneuploidy induced by T-antigen produces extra copies of the chromosome containing a factor that must be inactivated before immortalization occurs. Within immortalization-competent clones, the frequency of immortalization is about 3 X 10(-7). This frequency is consistent with a mutational mechanism for the inactivation of the factor that causes crisis in T-antigen expressing cells.