Thymic factors regulating DNA synthesis in lymphocytes cultured in vitro

An extract was prepared from the calf thymus. Thymic factors with stimulating or inhibiting effect on the incorporation of 3H-thymidine into DNA of cultured lymphocytes from guinea pigs and rats were identified and separated from each other by ultrafiltration, gel filtration, ion-exchange chromatography and electrofocusing. Two different stimulating substances were found both with molecular weight below 10,000 daltons. The result of ion-exchange chromatography and preparative electrofocusing indicated the existence of one (or two) acidic and one slightly alkaline stimulating thymic factor. The stimulating substances were found in greater amount in a young thymus than in a an old one. the thymic factors increasing the uptake of 3H-thymidine are probably polypeptides.