The sensitivity of 10 mouse lymphoid or myeloid cell lines to gamma-ray- and DNA-associated 125I-decay-induced clonogenic cell killing have been compared with their rate of loss of viability (membrane integrity) and with their putative cell type of origin. The pseudodiploid haematopoietic cell lines showed D0 values for 125I-induced DNA double-strand breakage (dsb) that ranged from 7.7 +/- 0.7 to 40.8 +/- 2.8 decays. These lines generally appeared to be more sensitive to killing by radiation-induced DNA dsb than are fibroblast-like cell lines. The increased sensitivity of haematopoietic cell lines to killing by DNA dsb may be related to their mode of death (apoptosis versus necrosis). Mode of cell death may thus be an important factor in determining the 'inherent radiosensitivity' of normal cells/tissues. Haematopoietic cell lines that undergo rapid interphase apoptotic death showed extreme sensitivity to DNA dsb. The latter cell lines were found to have derived from immature lymphoid cells, and it is speculated that their high radiosensitivity might reflect the action of a mechanism that normally eliminates cells containing illegitimate V(D)J recombinase-induced DNA dsb.