Thymic hormonal factors were isolated from mouse thymus by two methods. (1) Thymic cytosols in phosphate buffer saline were filtered through Sephadex G100 with 0.1 M NH4HCO3 (pH 8.0) as buffer and the protein peaks were collected. (2) Protein having thymosin activity (F5) was isolated from thymic cytosols after heat inactivation, salt fractionation and desalting on Sephadex G25. Molecular weights of all the proteins were determined on SDS-PAGE. Biological activity of thymic proteins was studied by in vitro and in vivo assays, using synthetic thymosin alpha 1 as the standard. Thymocytes treated with different thymic proteins showed maximum stimulation at 16 h of incubation period. Preincubation of the thymocytes with the thymic proteins and subsequent incubation with Con-A decreased the stimulation index. Incubation of spleen lymphocytes with thymic proteins increased the percentage of Tdt+ cells. The antitumor effects of thymic proteins carried out on animals having leukemia, showed statistically significant results. Clinically however, the antitumor effects of the thymic proteins alone and in combination chemotherapy were negligible at 1 mg/kg body weight dose level.