BACKGROUND Lymphocyte subset reconstitution was studied in 65 patients undergoing allogeneic and autologus bone marrow transplantation (BMT). METHODS The expression of molecules on the membrane of lymphocyte subsets was assessed by two-colour flow cytometry and a direct immunofluorescence assay. The functional capacity of the patient's T lymphocytes following transplantation was identified by stimulation whit peripheral blood lymphocytes; B cells from BMT recipients were tested for their ability to respond, in vitro, to pokeweed (PWD) mitogen. RESULTS 1) The proportion of CD8+ T lymphocytes was higher than the CD4+ T lymphocytes until 1 1/2 year after-BMT, with high percentage of immature T cells (CD3+, CD8+, HLA-DR+, CD25-) in the first nine months post-transplant. Moreover, a large proportion of T lymphocytes lacked CD5 expression in the first year following BMT. 2) T-cell proliferative response to PHA was low with subsequent recovery until normality. 3) Low numbers of B cells in the first two months with a significant increase since then until 1 1/2 year after-BMT; the phenotype of these B cells was mainly CD19+, CD5+. 4) High in vitro spontaneous immunoglobulin production by peripheral blood B lymphocytes and an impaired response to PWM was observed. 5) Increased percentage of cells with natural killer (CD56) cell phenotype was seen during the 2nd and 3rd months after the graft infusion. After 1 1/2 year postgrafting, this percentage returned to normal level. CONCLUSIONS Taken together, these data indicate the existence of numerous abnormalities in several subsets of peripheral blood lymphocytes after BMT and suggest a slow kinetics of immune recovery after human marrow transplantation being complete between 18 and 24 months following BMT.