A monochromatic, coherent, high-intensity laser beam, with a wave-length of 265 nm, was tested for its effect on cellular reproduction and nucleic-acid synthetic activity. Using cloning procedures, it was established that cellular reproduction rates of both the DON-CII and CMP cell-lines decreased in direct proportion to the imposed ultra-violet laser (UVL) dose. The dose-response curves suggested that both cell-lines were reacting to a single-hit phenomenon, but the human cell-line (CMP) was more sensitive than the hamster cell-line (DON-CII). ULV treatment of DON-CII cells inhibited both tritium and uridine uptake rates within the initial post-treatment period, but recovery occurred after 7 hours of incubation. However, an analysis of grain counts suggested that RNA metabolism was maintained at 75 per cent of control values from 7 to 24 hours after ULV treatment.