During the last 25 years it has become increasingly clear that short peptides derived from Ig V-regions are displayed on MHC class II molecules. Recognition of such idiotypic(Id)-peptide/MHC class II complexes by Id-specific CD4(+) T cells plays a role in (1) Id-driven T-B collaboration, (2) immunosurveillance of B cell cancers and (3) Id-vaccination. A crucial question is then: to what extent are T cells tolerized to Ig V-region sequences? Or rephrased: how large is the T-cell repertoire for Ig V-region sequences presented by MHC class II molecules? We argue that T cells are to a large extent tolerant to germline-encoded V-region sequences but that there is a T-cell repertoire for rare Id-sequences that arise as a consequence of somatic hyper mutation or N-region diversity. Moreover, when otherwise rare Id-sequences increase in concentration, T-cell tolerance is induced (Fig. 1). For these reasons, T cells that recognize rare Id-peptides, arising as a consequence of somatic genetic events unique to each B cell, may play a special importance in Id-driven T-B collaboration, immunosurveillance of B-cell malignancies, and Id-vaccination.