Autologous mixed lymphocyte reaction (AMLR) was studied in 37 patients with chronic ITP (including 27 active cases and 10 inactive cases) and 37 sex and age group matched healthy controls. AMLR was markedly decreased in all the patients of both active and inactive groups (P < 0.05). T cells and non-T cells from the peripheral blood of 10 patients with ITP and 10 Sex and age group matched healthy controls were investigated for HLA-DR expression by direct immunofluorescence. The percentage of DR+ non-T cells was significantly decreased in Patients with ITP as compared with the controls (P < 0.01) and there was a positive between AMLR and platelet counts (r = 0.92, P < 0.01; r = 0.75, P < 0.05 respectively). In addition, the percentage of DR+T cells was significantly increased in patients with ITP in comparison with the controls (P < 0.01). There was an inverse correlation between the percentage of DR+T cells and platelet counts (r = -0.82, P < 0.01) as well as AMLR (r = -0.81, P < 0.01). This study suggested that these were defects of the immunoregulatory function in patients with ITP as manifested by decreased AMLR, which might be related to the abnormal expression of surface DR+ antigen on non-T cells.