Nondividing human diploid fibroblasts maintained in medium containing 0.5% calf serum do not survive when exposed to low doses of UV (254 nm). The extent of killing is dose and strain dependent. DNA excision repair-proficient cells are more resistant than excision repair-deficient cells. Results of measurements of the effect of UV on RNA and protein synthesis in repair-proficient and -deficient (XP12BE) cells are reported. UV causes an immediate and equal depression of the RNA synthesis rate in both kinds of cells. A recovery to control rates was observed only at low (5 J/m2) doses in repair-deficient cells and at higher doses (20 J/m2) in repair-proficient cells. No recovery was observed at doses that cause substantial reductions in survival (greater than 5 J/m2 for XP12BE; greater than 40 J/m2 for repair-proficient populations). No initial effect on rate of protein synthesis was detected at doses less than 20 J/m2. However, in XP12BE populations, a decreased rate first evident at 15-30 h post-UV and before any cell degeneration and loss was observed for doses as low at 7 J/m2. This delayed effect was not observed in repair-proficient populations. The results are consistent with the hypothesis that the lethal action of UV in nondividing cells is one on DNA that leads to an inhibition of required protein synthesis by preventing RNA transcription.