Bone marrow transplantation is characterized by a prolonged period of humoral immunodeficiency in which many patients have abnormal circulating B-cell subsets, and oligoclonal and monoclonal gammapathies. In this study we examine B-cell precursor reconstitution in the post-transplantation marrow. Within 1 month after transplantation there is a marked increase in the percentage of immature B cells (to 80% of marrow lymphocytes), which can persist for more than 1 year. The increase in B-cell precursors is seen in both adults and children and appears to be independent of age. These cells have a normal precursor B-cell surface antigenic phenotype (CD19+, CD10+, CD20 negative to dim) and generally express very little CD34. No monoclonal or oligoclonal immunoglobulin gene rearrangements are detected in these cells, which enables them to be easily distinguishable from common precursor B-cell acute lymphocytic leukemia lymphoblasts.