A prospective study was undertaken to assess the nature, incidence and natural history of platelet antibodies in patients with immune thrombocytopenic purpura (ITP) and patients with onyalai, using an immunofluorescent technique. Twelve patients under 14 years old and 11 patients 14-75 years old with ITP, and 24 patients with onyalai were studied. Alternate younger patients were treated with corticosteroids. Ten of the 12 children with ITP had IgG platelet antibodies in their serum, which disappeared as the platelet count recovered. Steroid therapy did not change the course of the disease or the antibody response. Of the 24 patients with onyalai, 23 had IgG antibodies and 18 had IgM antibodies, which were still present after 14 days and unrelated to their rise in platelet count. Steroid therapy did not affect the platelet count or the antibody titre. The difference in immune response of ITP and onyalai points to a difference in aetiology. The clinical presence of IgM antibodies in onyalai fits the hypothesis that a toxin, possibly acting as a hapten, is responsible for this form of thrombocytopenia.