The activity and the percentage of suppressor T cells in PBMC from 28 patients with type I diabetes of various durations treated with thymosin or insulin were detected by using the method of ConA-induced suppressor T cells and Wu T8 (suppressor/cytotoxic) monoclonal antibody respectively. Both thymosin and insulin were found to have ability to improve and normalize the ConA-induced suppressor cell activity and the percentage of Wu T8 cells in diabetic patients. The maximal effects were found at the concentration of 3. 22?mol/L thymosin and 1. 722nmol/L(240. 05mIU /L) insulin. Thymosin was more effective than insulin.