The primary site for the growth and differentiation of B cell precursors in irradiated recipient mice was investigated. Bone marrow (BM) cells from lipopolysaccharide (LPS)-responder C57BL/6 mice were transferred into irradiated LPS-nonresponder C57BL/10ScCr mice, and the generation of donor-derived B cells in the recipient was monitored by determining the immunoglobulin-producing cells developed in the LPS-stimulated cultures of recipient's spleen cells as well as BM cells. As previously stated, the transfer of fresh BM cells resulted in the development of LPS-reactive cells both in spleen and BM simultaneously. On the other hand, when long-term cultured BM cells which were shown to be devoid of B cells and pre-B cells were used as the donor cells, the development of LPS-reactive cells was first observed only in BM, and subsequently in both BM and spleen. The failure to detect LPS-reactive cells in the spleen in the early phase, or day 11 after irradiation and reconstitution, was not attributed to the culture condition or the existence of suppressive activity in day 11 spleen cells. These results indicate that B cell precursors lodge only in the BM of irradiated recipients, grow and differentiate in the same place, and then the differentiated progeny migrate to peripheral lymphoid organs.