The sera of 14 patients with quinine/quinidine-dependent thrombocytopenia were studied in the platelet suspension immunofluorescence test (PSIFT), the 51Cr-lysis assay and the complement fixation test (CFT). When anti-Ig or anti-IgG reagents were used, the PSIFT proved to be a little more sensitive than the 51Cr-lysis assay and far more sensitive than the CFT. With the PSIFT, it was possible to determine the immunoglobulin class of the antibodies, which in all sera was IgG of the subclass IgG1. In two sera there were also IgG3 antibodies, and in the sera of three patients additional IgM drug-dependent platelet antibodies were detected. With the anti-C3 reagent, however, lower titres were observed in the PSIFT than in the 51Cr-lysis assay, in contrast to the results with other complement-binding IgG platelet antibodies. The F(ab')2 fragments of the antibodies did not react in the PSIFT. This indicates that the Fc part of the antibody is essential for the fixation on the platelet membrane. These findings support the theory that in this syndrome drug-antibody complexes area formed in he absence of platelets that selectively adhere to the Fc receptor on platelets. The platelets of patients with the Bernard-Soulier syndrome (BSS) reacted normally with the quinine/quinidine-dependent antibodies in some of the sera, but not with those in other sera in the PSIFT. However, lysis of BSS platelets was never observed. The results of absorption studies showed a reflection of the results in the PSIFT, indicating that the receptor for the quinine/quinidine-dependent antibodies is not absent in this disease but altered. Whether a reaction with BSS platelets will take place depends on characteristics of the antibodies.