In an attempt to quantitate DNA repair induced by Neocarzinostatin, the incorporation of labeled thymidine into the human peripheral lymphocytes after exposure to the agent was studied by autoradiography. The exposure of lymphocytes to Neocarzinostatin for 60 min at the concentrations as low as 0.05 mug/ml caused a significant increase in the number of grains per cell within 2 hr of incubation with 10 muCi/ml of 3H-thymidine in the presence of 2mM of hydroxyurea. The number of grains increased with the increasing dose of the agent up to the concentration of around 2.5 mug/ml and then fell at higher concentrations. The extent of maximum incorporation induced by Neocarzinostatin was found to be almost comparable to that induced by 100 erg/mm2 of UV irradiation. Hydroxyurea or caffeine in the labeling medium showed little or no effect on the grain count, but acriflavine at 0.1 mM reduced the grain count by a factor of about 6. These results indicate that relatively high level of repair synthesis occurred in human lymphocytes after exposure to Neocarzinostatin and provide further evidence for the direct damage of cellular DNA induced by the antibiotic.