By applying the platelet suspension immunofluorescence test (PSIFT), platelet-specific alloantibodies responsible for post-transfusion purpura were detected in eight patients within a period of 2 years. All patients were female and had previously received blood or had been pregnant. The platelet-specific alloantibodies had the specificity anti-Zwa in all the patients, who were all Aw(a-). In two patients the platelets were tested in the acute phase of the disease and found to be coated with IgG. In one patient an ether eluate was prepared from the platelets that reacted strongly with Zw(a+) platelets, but weakly with platelets from Zwb-homozygous individuals. The sera of these two patients, and of two others whose platelets were not directly tested, taken in the acute phase of the purpura, reacted strongly with Zwa-positive platelets. The four sera also reacted, however weakly, with Zwa-negative platelets, with autologous platelets taken during remission and with platelets from patients with Glanzmann's disease. It is postulated that Zwa-anti-Zwa complexes, present in the eluate and the sera, caused the reaction with Zwa-negative platelets and the patients' own platelets. Immunochemical characterization of the post-transfusion purpura antibodies showed that in all patients these were IgG, in two combined with IgM antibodies. Antibodies of the sub-class IgG1 were found in all patients, sometimes together with IgG3. In the indirect immunofluorescence test with anti-complement serum, the PTP antibodies in only four sera were able to fix complement. In only two of these sera were these complement-binding antibodies detectable in the 51 Cr-lysis technique and then in a much lower titre than in the immunofluorescence technique.