We established T cell clones, which were considered to be the possible cause of transfusion-associated graft-versus-host disease (TA-GVHD), from the peripheral blood lymphocytes (PBLs) of two patients. In both cases, several CD4+ cytotoxic T-cell (CTL) clones were established. In case I, the target antigen of the established CD4+ clones was a DRB1*0403-related antigen serologically typed as HLA DR4, which was one of the patient HLA antigens. In case II, the target of four out of five established CD4+ CTL was a DRB1*1302-related antigen. One CD4+ CTL clone showed cytotoxicity against cells carrying A*2402, B*4403, Cw*1403 and DPB1*0401. A monoclonal antibody (mAb) blocking study showed only anti-DP mAb inhibited the cytotoxicity of this clone. Thus, it might be considered that this clone recognizes HLA-DP with its binding peptides derived from either A*2402, B*4403, Cw*1403 or DRB1*1302. Our findings indicate that CD4+ CTLs may play important roles in the aetiology of TA-GVHD and that the antigens of patients recognized by donor-derived effector cells may not always recognize a single HLA antigen.