In 15 patients with thrombocytopenia EDTA-dependent platelet antibodies (IgM agglutinins active in 20 degrees C) were detected. These antibodies were found in patients with autoimmune diseases, infections and neoplasma as well as in healthy persons (including pregnant woman). In 10 persons pseudothrombocytopenia (PTCP) was diagnosed since the low platelet counts were found only in EDTA-blood, and the patients did not have bleeding symptoms. In 5 other cases EDTA-dependent antibodies were not the only cause of thrombocytopenia since the low platelet counts were also observed in the citrate-blood and occassionally the bleeding symptoms occurred; in 3 of them in addition autoantibodies were suspected. Due to the proper diagnosis of PTCP, in five patients unnecessary corticotherapy was discontinued, one person avoided splenectomy, in two patients previously postponed surgery was performed and one person could be passed for employment.