Previous reconstitution studies in irradiated mice distinguished Ly-1 B from other B cells. The predominant B cell populations in spleen and lymph node are readily reconstituted from progenitors present in adult bone marrow. Ly-1 B cells, in contrast, are reconstituted from cells in peritoneum rather than bone marrow. Further studies, presented here, demonstrate that Ly-1 B populations are reconstituted from peritoneal cells that already express both surface immunoglobulin (IgM) and Ly-1, that is, Ly-1 B. Such cells reconstitute and maintain the Ly-1 B population in irradiated recipients for at least 6 months.