Binding of [3H]SR 27417 to washed rabbit platelets was time-dependent and saturable. [3H]-SR 27417 binding was reversible after short incubation periods but became progressively irreversible when incubated for more than 2 hr. Scatchard analysis of the saturation binding data indicated that [3H]-SR 27417 bound to two populations of specific binding sites with high (KD = 0.75 +/- 0.06 nM; Bmax = 58.7 +/- 4.3 fmol/10(8) cells; N = 3) and low affinity (KD = 53.8 +/- 4.9 nM; Bmax = 1665 +/- 87 fmol/10(8) cells; N = 3). Unlabelled C16-PAF competitively and selectively inhibited the specific binding of [3H]SR 27417 with an IC50 value of 1.9 +/- 0.04 nM (N = 3). SR 27414 fully and competitively displaced [3H]SR 27417 from its binding sites on rabbit platelets with a Ki value of 260 +/- 20 pM (N = 3) therefore demonstrating that SR 27417 was more potent than C16-PAF itself. On washed human platelets, [3H]SR 27417 displayed specific as well as saturable binding to two populations of binding sites (KD = 0.21 +/- 0.01 nM; Bmax = 13.9 +/- 0.9 fmol/10(8) cells and KD = 4.75 +/- 1.9 nM; Bmax = 82.2 +/- 2.9 fmol/10(8) cells; N = 3) for high- and low-affinity binding sites, respectively, whereas [3H]SR 27417 bound to only one single class of binding sites on human polymorphonuclear leukocytes (KD = 0.31 +/- 0.1 nM; Bmax = 9.36 +/- 1.2 fmol/10(6) cells; N = 3). IC50 values for C16-PAF, SR 27417 and other PAF receptor antagonists on all three cell types indicated that SR 27417 was at least three times more potent than C16-PAF itself and more than 30-fold as active as the best synthetic PAF receptor antagonist tested (L 659,989). In conclusion, these data indicate that SR 27417 appears to be one of the most potent PAF receptor antagonists yet described, as well as a suitable radioligand for labelling PAF receptors on intact blood cells.