Argument exists as to whether platelet damage in chronic idiopathic thrombocytopenic purpura (ITP) is mediated by an autoimmune response to platelet antigen or by immune complexes. We have studied thirty-nine patients with ITP for evidence of (i) sensitization to platelet antigen, using a macrophage migration inhibition factor (MIF) assay, (ii) circulating immune complexes, using a Clq deviation technique, and (iii) serum-induced platelet 'immunoinjury', using a 3H-serotonin release assay. Eighty-one per cent of the patient group had a migration index of less than 0.8 (normal range 0.8-1.3), while 91% of those tested had a serum factor (presumably immune complexes) which bound to labelled Clq. The serotonin release assay was abnormal in 32% of twenty-two patients, and the results of this test bore no clear relationship to either those of the Clq deviation test or the MIF results. We conclude that most patients with ITP are sensitized to platelet-associated antigen, and have circulating immune complexes. The contribution of these factors to platelet destruction and the nature of the antigen in the complexes remain to be demonstrated. The serotonin release assay is less sensitive, non-specific, and has limited value in the diagnosis of ITP.