The recovery of the B-cell population in adult thymectomized, irradiated and bone marrow-reconstituted mice (T X BM mice was estimated at various times after bone marrow transplantation. The spleen cells to be tested were mixed with dexamethasone-resistant thymocytes (DRT) and sheep red blood cells (SRBC) and transferrred to irradiated recipients. The number of plaque-forming cells (PFC) in the spleen of the recipients was determined 7 days later. Using this functional B-cell assay a sequential appearance of the precursors of IgM-, IgG- and IgA-PFC in the spleen of T X BM mice was observed. The precursors of IgM-PFG (IgM-B cells) were present immediately after transplantation. The first IgG-B cells could be detected at 13-16 days after transplantation and the IgA-B cells finally appeared at 22 days after transplantation. The number of B cells reached a constant and normal level at 30 days after transplantation. The IgM-, IgG- and IgA-B cell development in sham-thymectomized, irradiated and bone narrow-reconstituted mice (ST X BM mice) was virtually the same as in T X BM mice.