40 patients with idiopathic thrombocytopenic purpura (ITP) were studied for T-cell abnormalities using a panel of monoclonal antibodies (McAbs). These patients showed a tendency to have a decreased OKT4+:OKT8+ ratio compared with normal subjects and a significant increase was observed in OKT10-positive cells in ITP patients. Further analyses were made utilizing McAbs of different subclasses in combination (BMA040 mouse IgG1 McAb, helper/inducer T and BMA081 mouse IgG2 McAb, suppressor/cytotoxic T). An increased proportion of cells reactive with both BMA040 and BMA081 McAbs (double-labelled cells) was demonstrated in patients with ITP compared with the normal controls. Furthermore, there was an inverse correlation between the proportion of "double-labelled cells" and the platelet counts. These data suggest that abnormalities of the T-cell subsets in the peripheral blood may play an important role in the pathogenesis of ITP.