By lowering the pH of the medium, allo- and autoantibodies bound to granulocytes and platelets could be eluted from these cells and their presence in the eluate could subsequently be detected with normal donor granulocytes and platelets, respectively, in the immunofluorescence test and/or agglutination test. IgG- as well as IgM- and IgA-containing antibodies were detectable in the eluates. Depending on the attraction mechanism between antigen and antibody, some antibodies were also eluted by dimethylsulphoxide at pH 9.5, ether and/or by heating at 56 degrees C. By differential elution of antibodies from donor platelets, we demonstrated the simultaneous presence of platelet-specific antibodies and HLA antibodies in the serum of a mother whose newborn suffered from transient neonatal thrombocytopenia.