The mouse B-cell repertoire develops from a restricted set to an extremely diverse set of specificities. Establishment of some of the B-cell clones that constitute the adult repertoire is facilitated and guided by idiotype-directed interactions among complementary sets of B cells early during ontogeny. Through in vivo experiments described here and reported elsewhere, we have shown that the program of B-cell development involving idiotypic interactions is obligatory in the development of certain B cells that provide immunity against bacterial infections. Furthermore, this program of B-cell development is facilitated in newborn mice and not in adult mice that have been transplanted with progenitor cells from adult bone marrow. Thus the idiotype-directed selection of the adult B-cell repertoire appears to be limited to fetal-neonatal stages of development.