The method described in this paper can be used for detecting anti-platelet iso-, hetero-, and drug-antibodies and for detecting a factor in the plasma of patients with idiopathic thrombocytopcnic purpura that attaches to platelets (ITP factor). Plasnia from thc test subject, anticoagulated with citrate, is added to a suspension of normal platelets labelled with ['4C]~er~tonin and the aniount of radioactivity (serotonin) released is measured after incubating the mixture for 45 min at 37°C. Anti-platelet antibodies and drug antibodies that fix complement as well as isoantibodies not dctectable in vitro by complement fixation or agglutination can be measured by this test. Plasma from 24 of 40 patients with ITP released significantly more serotonin than control plasmas (P< 0.025). False positive results were obtained in less than 2.5 "/, of plasmas from normal individuals, from patients with a variety of diseases and from patients with secondary thrombocytopenia. However, plasmas from two of 13 patients with systemic lupus erythematosus and normal platelet levels gave positive results in the test. The 7s gamma globulin fractions of plasmas containing isoantibodics or ITP fictor as well as acid eluates from platelets which had been incubated in these plasmas gave positive serotonin-release tests.Although many anti-platelet isoantibodies and most drug antibodies responsible for platelet destruction in vivo can be measured by either complement fixation or platelet agglutination techniques (Shulman, 1958;Shulman et al, 1964)~ some isoantibodies do not react in these convcntional tests (Shulman, 1966). The non-complement-fixing, non-agglutinating isoantibodies in some instances can bc measured by their ability to interfere with or 'block' the complement-fixing activity of another antibody with the same antigenic specificity (Shulman et al, 1964). However, 'blocking' tests usually cannot be perfornicd because the specificity of the antibody is not known. In many respects, the humoral factor found in the plasma

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SummaryTwo sensitive methods for the detection and characterization of platelet isoantibodies are presented in this paper. The methods involve incubating platelets with either radioactive serotonin or adenine, interaction of the platelets with serum antibody and finally detection of supernatant radioactivity. Both tests can be used for HL-A typing of platelets and both tests detect antibodies undetectable by complement fixation. The requirement of a heat labile serum component is variable in the serotonin test but absolute in the adenine test. Antibody activity was found to be in the 7S gamma globulin fraction on DEAE cellulose column chromatography.