Seven clonally derived human amniocyte cell lines from independent Simian virus-40 transformation events were analyzed for chromosomal aberrations in the precrisis stage. The distribution of chromosomal aberrations over the haploid complement was nonrandom. With increasing passage number the number of involved chromosomes increased, but the type of aberrations were the same: dicentric chromosomes and translocations due, respectively, to fusion of telomeres and breakage in the centromere regions. Interruptions to telomeres were also evident from extrachromosomal material terminally on the p arms, especially, of chromosomes #7 and #11. The cytogenetic similarities between the present results and those in transfection experiments are mentioned. All seven clones became highly heterogeneous over 10 to 20 passages (i.e., 30-60 population doublings) when, in general, transformed amniocytes enter crisis. The chromosomes which were nonrandomly involved in aberrations exhibited great diversity in types of rearrangements. Thus, for each clone multiple different rearrangements for specific chromosomes were observed in different cells. Most of these aberrations had a single step origin, which implies that breakage occurred repeatedly for the same chromosome in different cells, suggesting that the specific chromosome had become unstable in an earlier cell generation. Thus, we observed development of clonal chromosome instability, a phenomenon not reported for viral transformed cells.