Antibodies against megakaryocytes into the bone-marrow were investigated with the aid of direct immunofluorescence in 56 children with chronic-idiopathic thrombocytopenic purpura (ITP) and in 10 children of a control group. The megakaryocyte concentrate of the bone-marrow was produced by using a density grade separation with percoll. In children of the control group, small amounts of fixed immunoglobulins of all three main classes (G, A and M) could only be found on the membrane of megakaryocytes. It was only in some cases that illuminating granules could be identified on the membrane. In patients with ITP the number of megakaryocytes with IgG fixed to the membrane was 10 times higher than in the control group. Immunofluorescence was evident and could be seen to cover part of the cell periphery or even the whole of it. By using an effective hormone therapy the number of megakaryocytes with IgG fixed to the membrane could be shown to have a tendency to diminution, attaining its control level, however, only after splenectomy. The findings obtained point to the possibility of forming antibodies directed against megakaryocytes and they emphasize the possibility of an immunopathologic trouble of thrombopoiesis in chronic ITP.