Peripheral blood mononuclear cells from a DQW-1 homozygous individual were cocultured with irradiated lymphoblastoid cell line from a DQW-1 homozygous unrelated donor bearing BW35-DW1 haplotype. From T cell cloning of primary and twice-stimulated mixed leukocyte cultures (MLC), 7 and 11 T cell clones were obtained respectively. None of the 18 clones showed specific cytotoxic activity against the alloantigen of the stimulator cell as well as natural killer (NK)-like activity against K562 cells. However, most T cell clones from both primary and re-stimulated MLC demonstrated moderate cytotoxic activity in lectin-dependent cell-mediated cytolysis (LDCC) assay. Screening assay for cell-mediated lympholysis (CML) performed on growing microcultures obtained from restimulated MLC cloning confirmed the non-cytotoxic status of these T cell clones by showing that 41 out of 44 growing microcultures were not cytotoxic against the stimulator cell; the other 3 clones lyzed the target cell mildly. The cells from all 5 T cell clones detected for indirect fluorescence expressed CD3 and CD4 surface markers. Taken together, the results suggested that proliferation-regulating T cell subsets or factor(s) may be generated during the course of MLCs under the present responder-stimulator combination, and may suppress the development of alloreactive cytotoxic T cells and NK-like cells.