Using immunoglobulin (Ig)-recognizing T helper clones the expression of Ig peptide/major histocompatibility complex class II complexes derived by the processing of endogeneous Ig molecules in the thymus was demonstrated. It was found that thymic B cells but not "classic" thymic antigen-presenting cells and macrophages represent the major antigen-presenting cell type of determinants of endogenously synthesized surface Ig (Ig kappa-1b) and anti-surface Ig antibodies (IdC3B9). The Ig kappa-1b-presenting activity in the thymus appears relatively late, only after 3 weeks of postnatal life, while in the spleen an efficient presentation of endogenous Ig kappa-1b epitope is observed very early after birth. This difference between thymic and peripheral presentation of endogeneous Ig determinant could be important for understanding the mechanisms of T cell tolerance to self Ig and the role of self Ig in negative and positive selection of T cell repertoire.