Lens refractive function was monitored during long-term bovine lens culture experiments in which low concentrations of potentially damaging agents were added to the culture media. The agents tested were the drugs DL-propranolol and prednisone, and hydrogen peroxide. A computer-driven scanning laser system was used to monitor lens focal length during culture. The system consists of a scanning helium-neon laser beam, a television video camera, and a video frame digitizer. The system first locates the optical center of the lens, defined as the position of little or no refractive deviation of the beam. The laser scans the lens in small steps (0.05 mm), while the digitizer measures focal length for each beam position. A graphic profile of lens focal variation is plotted. A concentration of 0.1 mM hydrogen peroxide produces no significant change in lens focus after 315 hr of incubation. When the concentration is increased 10-fold (1.0 mM), lens focal ability is radically disturbed after 60 hr of incubation. Lenses incubated with prednisone (0.03 mM) for up to 525 hr show no difference in focal characteristics in comparison to control lenses. Propranolol in a concentration of 0.1 mM disrupts lens focal ability after 250 hr of incubation, whereas a higher concentration (1.0 mM) produces a similar effect after only 150 hr. These results indicate that lens refractive function is a sensitive measure of lens function during culture.