For the detection of auto-antibodies on the platelets and in the serum of patients with idiopathic thrombocytopenic purpura, three new techniques have recently been developed: the quantitative antiglobulin consumption assay (QACA), the platelet radioactive antiglobulin test (PRAT) and the platelet suspension immunofluorescence test (PSIFT). The results obtained by various investigators with these techniques differ considerably. We, therefore, studied the sensitivity of the three methods. This was done by testing platelet-reactive allo-antibodies (anti-Zwa, anti-HLA-A2) and auto-antibodies in titration. The results show that the PSIFT is the most sensitive technique, closely followed by the PRAT. The QACA was found to be much less sensitive than the other two methods. This suggests that a positive result in the QACA and a negative result in the PSIFT and/or PRAT cannot be attributed to the presence of platelet auto-antibodies.