The immune mechanisms of T cells regeneration after bone marrow transplantation (BMT) and the factors maintaining allogeneic marrow graft in the host are still unknown. To pursue this issue, we analyzed T-cell clonality of peripheral blood lymphocytes (PBLs) in BMT recipients, using reverse transcription polymerase chain reaction with T-cell receptor (TCR) V beta gene segment-specific primers and single-strand conformation polymorphism. PBLs from patients and donors showed a heterogeneous T-cell population with oligoclonal accumulations of CD8+ T cells. When PBLs were cultured in HLA-matched mixed lymphocytes reaction in vitro, no distinct clonal expansion was observed. However, after BMT, oligoclonal expansions were induced in the recipients in vivo, without a restriction of TCR V beta gene usage. Although part of the expansion was transient, the majority was repeatedly detected even several months later. Our results suggested that certain in vivo mechanisms maintain a stable clonal expansion of distinct T cells in marrow recipients. We also found in a single patient with graft-versus-host disease a replacement of expanded clones by other clones during follow-up. Diminishing numbers of accumulation clones were found in long-term marrow recipients, indicating a general tendency for clonal expansion to subside progressively. Considered together, our data suggest the involvement of clonally expanded T cells in lymphoid regeneration and in acute and chronic immune responses after BMT.