Serial assessment of peripheral blood T and B cell recovery and serum immunoglobulins was performed in 19 children for the first year following BMT and compared with normal values established from healthy children. Immunophenotypic analysis on bone marrow was performed in selected cases by Southern blotting of the immunoglobulin heavy chain (IgH) gene. We found no significant differences between T cell-replete or depleted allogeneic bone marrow transplants. Lymphocyte numbers were low until 9 months post-BMT. T cell numbers (CD2, CD3, CD5) were also low until 12 months but B cell numbers (CD19) became normal at 3 months. Both CD4+ and CD8+ T cell subsets were low post-BMT with depression of CD4+ greater and more prolonged than that of CD8+. No overshoot of CD8+ was seen. The principal effect of GVHD or its treatment was further depression of CD4+ cells but with no increase in CD8+; recovery of B cells was also delayed. Recovery of IgG was slow with only six of 11 children reaching an age-adjusted normal level by 1 year, whereas there was more rapid recovery of IgM and IgA. Several children had an increase in lymphocytes of immature appearance in their bone marrow at varying times post-BMT with increased cells of phenotype CD19+, CD10+, HLA-DR+ and TdT+. In each case Southern blotting showed a germline pattern of the IgH indicating a polyclonal early B cell regenerative population.