Yangambin, a new naturally-occurring platelet activating receptor (PAF) receptor antagonist competitively displaced [3H]-PAF from its high affinity binding sites on washed human platelets with a Ki value of 1.1 +/- 0.3 microM (n = 3). Studies carried out in parallel demonstrated that SR 27417, a newly-developed PAF receptor antagonist also antagonized [3H]-PAF binding to these cells with a Ki value of 51 +/- 2 pM. SR 27417 (N-(2-dimethylamino ethyl)-N-(3-pyridinyl methyl) [4-(2,4,6-triisopropyl phenyl) thiazol-2-yl] amine) selectively and competitively inhibited the specific binding of [3H]-PAF on human polymorphonuclear leukocytes (Ki = 65 +/- 5.2 pM) whereas high doses of yangambin remained ineffective. Yangambin inhibited PAF-induced aggregation of human platelets in vitro (IC50 = 1.0 +/- 0.2 microM) but had no effect PAF-induced oxidative burst in human polymorphonuclear leukocytes. In guinea pigs, yangambin inhibited PAF-induced thrombocytopenia but did not affect leukocytopenia whereas SR 27417 afforded complete protection against both PAF-induced thrombocytopenia and leukocytopenia. In conclusion, yangambin discriminates between two different types of PAF receptors on platelets and polymorphonuclear leukocytes and can be considered as the first PAF receptor antagonist described to date exhibiting such an effect.