The purpose of this study was to characterize the phenotype and clonality of the T cell population in patients who experience acute rejection (AR) following bone marrow transplantation (BMT) from a partially mismatched related donor (PMRD). Phenotypic analysis was performed using flow cytometry, assignment of donor/host lineage by cytogenetics or HLA-specific flow cytometry, and analysis of the T cell receptor (TCR) by reverse-transcriptase polymerase chain reaction (RT-PCR). We have previously reported the initial appearance in the blood of AR patients of host CD8+(bright) CD3(low) T cells that progressively express increasing amounts of CD3+ cells. We now report that this cell population can differentiate into either a cytotoxic T cell phenotype (CD3+CD8+HLA-DR+CD57-) usually associated with AR of grafts from matched unrelated donors or a suppressor T cell phenotype (CD3+CD8+CD57+HLA-DR-) usually associated with AR of grafts from matched sibling donors. Analysis of the TCR Vβ subsets from two patients revealed sorted host CD3+CD8+ cells (purity 90-95%) from the first patient to express Vβ18 almost exclusively. In a second patient with late rejection (55 days post-BMT), the CD3+CD8+ cells were predominantly restricted to Vβ 1, 5.1, 7, 9, and 18. Although CD3+CD8+ T cells are known to be associated with AR, cytotoxic and suppressor lineages in AR from the same type of BMT and clonal distribution of T cells in AR have not been reported. Preliminary results suggest that Vβ expression in AR of PMRD grafts is restricted and host T cell phenotype may vary. Further studies will investigate whether specific mismatches correlate with specific Vβ usage and/or host T cell phenotype.