Studies on the maturation of the B-cell population are proposed. The role of the thymus in regulating the maturation of the function of the B-cell population will be studied. Three aspects of B-cell function will be evaluated: (1) ontogeny of the capacity to produce a heterogeneous response to a T-independent antigen; (2) ontogeny of the capacity to resynthesize surface immunoglobulin after capping with anti-immunoglobulin; (3) the ontogeny of the resistance to tolerance induction which is typical of mature B cells. The studies will be carried out using cell transfer methods to evaluate the function of B cells from donors of various ages transferred into lethally irradiated adult recipients. The influence of thymus cells on B-cell maturation will be evaluated.