The diversification of the receptors of B and T lymphocytes is determined by stepwise gene rearrangement of the V genes and the D-J segment. However, this repertoire is not yet selected within the host environment and thus is referred to as the potential repertoire. The repertoire of the peripheral mature B and T-lymphocytes is distorted and selected from the potential repertoire by various auto- and foreign antigens in the host environment and is referred to as the available repertoire. It is well documented that the peripheral T-lymphocyte repertoire is first selected in the thymus to delete or inactivate self-reactive clones. Moreover, extensive studies done during the last decade suggest that the B- and T-lymphocyte repertoires are significantly influenced by the host immunoglobulin idiotype repertoire. In this review, the role of immunoglobulin idiotypes in the selection of the B- and T-lymphocyte repertoires is discussed.