Bone marrow graft recipients suffer profound immunodeficiency during at least 3 months after transplantation. B-cell reconstitution following allogeneic bone marrow transplantation (BMT) in children was studied longitudinally by quantification of Ig (sub)class levels in serum and by investigation of numbers and characteristics of homogeneous Ig components (H-Ig); ie, monoclonal gammopathies (MG). For the latter purpose, a sensitive immunoblotting technique capable of detecting H-Ig of a concentration as low as 0.5 microgram/mL was used. Sera of 40 children grafted for a variety of diseases were investigated and followed up for 5 years. It was found that Ig (sub)classes reached normal levels from 3 months after BMT onward. The sequential increase of the different Ig isotypes was in accordance with that seen in normal ontogeny. This was especially clear following BMT for severe congenital immunodeficiency. H-Ig appeared from as early as 6 weeks after BMT in increasing numbers, beginning within IgM, IgG3, and IgG1, and afterward within other isotypes. After an initial increase of serum Ig levels, "overshooting" occurred accompanied by high frequency of H-Ig. H-Ig were still present at 5 years after BMT, when Ig levels normalized. Our data indicate that B-cell reconstitution after allogeneic BMT recapitulates normal ontogeny but in a clonally dysregulated fashion; that is, with overexpression of some clones and underexpression of others.