Chromosomal instability in proliferating mammalian cells is characterized by a persistent increase of chromosomal aberrations and rearrangements occurring de novo during successive cell generations. Recent results from many laboratories using a variety of cells and cytogenetic end points show that this phenotype can be induced by low as well as high LET irradiation. A typical feature of chromosomal instability in primary human G0-lymphocytes exposed to gamma-irradiation at both high dose rate (45 Gy h-1) and low dose rate (0.024 Gy h-1) is the appearance of novel aberrations in the clonal progeny of the irradiated cell, many generations after the exposure. The same phenotype was observed in lymphocytes that were allowed to recover for 5 days in G0 after the radiation exposure, as well as in hprt-mutant T cell clones. These results demonstrate that neither the acute genotoxic stress caused by high dose rate as compared to low dose rate irradiation, nor a hypothesized conflict between mitogen induced growth stimulation and growth arrest due to radiation damage, seem to be critical conditions for the development chromosomal instability in these cells. In contrast to observations in other cells, no evidence of a persistent decrease of cloning ability was observed in the progeny of radiation-exposed human lymphocytes, and no alteration was observed in their sensitivity to a second radiation exposure. Furthermore, the frequency of CA-repeat length variation at three loci was not increased in the progeny of X-irradiated T cells as compared to non-irradiated cells, which indicates that microsatellite instability is not part of the chromosomal instability phenotype in human T-lymphocytes.