A serological follow-up study was undertaken in 47 patients with bone-marrow failure, who were repeatedly transfused with random donor granulocytes and/or platelets. Sera, obtained at regular intervals, were investigated in the leucoagglutination test, the lymphocytotoxic test and the immunofluorescence test on paraformaldehyde-fixed platelets, granulocytes and lymphocytes. The frequency of alloimmunization was high (73%). Not only HLA antibodies, but also blood-cell-specific alloantibodies were detected in the sera of the alloimmunized patients, e.g. lymphocyte-specific, platelet-specific and granulocyte-specific antibodies. The immunofluorescence test on platelets was also used as a crossmatch, and when this test was positive it was always found that after platelet transfusion the increment value was nil.