In rats, the glycoprotein Thy-1 is expressed on recently bone marrow (BM)-generated B cells but not on mature recirculating follicular (RF) B cells. Here we demonstrate that Thy-1(+) B cells consist of two phenotypically distinct, but developmentally related, populations: a population of newly formed (NF) B cells (IgM(br)-IgD(du)) that give rise to the second, less immature, Thy-1(+) population of so-called early recirculating follicular (ERF) B cells (Thy-1(+)IgM(du)IgD(br)) cells. These cells ultimately develop to RF-B cells (Thy-1(-)IgM(br)IgD(du)). Kinetic studies reveal that in absolute numbers per day most cells die at the transition of NF-B cells in the BM and those in the periphery: less cells die at later stages of B cell differentiation. Given the notion that this cell loss is not random, we speculate that NF-B cells and ERF-B cells may represent crucial steps during peripheral B cell development and their selection. Identification of their unique phenotype makes it possible to evaluate their roles in development of the antibody repertoire. (C) 1995 Academic Press, Inc.