Plateau-phase A549 cells exhibit a high capacity for repair of potentially lethal radiation damage (PLD) when allowed to recover in their own spent medium. Addition of either insulin or insulin-like growth factor-1 (IGF-1) to the spent medium 60 to 120 min before irradiation significantly inhibits PLD repair. The 9-h recovery factor (survival with holding/survival without holding) is reduced from 10.8 +/- 0.7 to 3.4 +/- 0.3 by insulin and to 3.0 +/- 0.4 by IGF-1. Neither growth factor alters the cell age distribution of the plateau-phase cells, increases the rate of incorporation of 5-bromo-2'-deoxyuridine into DNA, or alters the extent of radiation-induced mitotic delay in cells subcultured immediately after irradiation. Both insulin and IGF-1 alter the kinetics for rejoining of DNA double-strand breaks (DSBs), slowing the fast component of rejoining significantly. However, these growth factors have no effect on the initial level of DSBs or on the percentage of residual unrejoined breaks at 120 min postirradiation. Both growth factors affect repair of lesions leading to dicentric, but not to acentric, chromosome aberrations significantly. In control cells (treated with phosphate-buffered saline, 90 min prior to irradiation), the half-time for disappearance of dicentrics was 4.1 h (3.4 to 5.1 h), and 47.1 +/- 3.7% of the residual damage remained at 24 h postirradiation. Insulin and IGF-1 increased the half-time for disappearance of dicentrics to 5.2 h (3.9 to 7.7 h) and 5.7 h (5.5 to 5.9 h), respectively, and increased residual damage to 56.1 +/- 5.9% and 60.8 +/- 6.0%, respectively. Overall, these data show that insulin and IGF-1 inhibit PLD repair in A549 cells by mechanisms which are independent of changes in cell cycle parameters. The data suggest that the growth factors act by inducing changes in chromatin conformation which promote misrepair of radiation-damaged DNA.