The mammalian equivalent of the Bursa of Fabricius, the organ responsible for B cell maturation in avian species, has not been identified despite anatomic and ablative studies which suggest that the gut-associated lymphoid tissues (GALT) subserve this function. By analogy to the Bursa, the mammalian organ directing B cell ontogeny should be the site where IgM-bearing cells (B cells) are first identifiable. In this study, fluorescein-tagged heavy chain specific antirabbit IgM is used to localize initial sites of B cell appearance in rabbit fetal and neonatal lymphoid tissues. IgM-bearing cells are found 2 days before birth in the thymus and 1 day before birth in GALT. Immunoglobulin-bearing cells in spleen, lymph node, and bone marrow are undetectable until after birth. B cells bearing the IgM marker precede the appearance of IgG-bearing cells by 1 to 4 days in all instances. Intraperitoneal implantation of Millipore chambers containing immature fetal thymic tissue into neonatal hosts reveals that in situ development of IgM cells takes place independent of host cell traffic. The results suggest that B cell ontogeny in mammals is more complex than in avian species and demonstrates probable involvement of the thymus in the maturational process.