A study was made of the influence of transforming growth factor beta (0.05-50.00 ng/ml) on the intensity of 3H-thymidine incorporation in the DNAs of pseudonormal mouse fibroblasts of NIH 3T3 line, of cells of NRK-49F line from normal rat kidney, and of cells of A-549 line from human lung adenocarcinoma. The experiments were carried out in the absence and in the presence of epidermal growth factor (5 ng/ml), and(or) insulin (1 micrograms/ml), as well as in the presence of different concentrations of fetal calf serum, and while using different time of incubation of cells with the above mentioned growth factors. It was shown that depending on the culture conditions the transforming growth factor beta exerted stimulatory, inhibitory or no action on the intensity of DNA synthesis in the cells of the same type. An attempt was made to analyse the reasons, which may significantly determine the direction of regulatory influence of the transforming growth factor beta on DNA replication in the cells.