The immunological capacity of lymphoid cells from mice rendered tolerant to high and low doses of BSA was investigated. The tolerance was induced by multiple injections of high and low doses of antigen through the period of 30 days. Lymph node and bone marrow cells from tolerant animals were transferred into lethally irradiated syngeneic recipients. After 10 days, when lymphoid organs of the recipients were repopulated with the injected cells, challenge injection of the same antigen incorporated into complete Freund's adjuvant was given. The immune response of the transferred cells in the recipients was evaluated by analysis of the specific antibodies in the sera. Lymphoid cells from donors rendered toloerant with high doses of antigen recovered their reactivity 20 days after the transfer to the level of reaction of normal cells. Lymphoid cells from donors receiving multiple injection of low doses of BSA remained tolerant after the transfer through the entire observation period. According to the cellular events in the donors during the period of tolerance induction, and the behaviour of the transferred lymphoid cells in the new recipients, it seems possible that tolerance induced with high doses of BSA corresponded to the B-cell tolerance, while low doses of antigen most likely induced tolerance of T-cell population. The possible cellular mechanisms of B and T-cell tolerance were discussed.