Specific binding sites for ANF have been identified on human platelets. To determine maximal binding (Bmax) and dissociation constant (Kd), we adapted the only original method by developing a specific sequence of platelet preparation. From venous blood collected on citrated anticoagulant, platelets were prepared by successive centrifugations at 20 degrees C (blood centrifugated at 1500 rpm for 10 min., supernatant centrifugated at 3000 rpm for 1 min., supernatant centrifugated at 2800 rpm for 10 min, the inner platelet-rich layer resuspended in citrated solution) and aliquoted (200 microliters at 5.10(5) platelets/microliters). Competition experiments [incubation of platelets with fixed concentration (20-25 pM) of labeledhuman ANF (125Ih ANF) and increasing concentrations (10(-12) to 10(-6) pM) of unlabeled hANF] led to the drawing of a mean displacement curve (n = 8), usable as reference, and to verification of the specificity of binding assay (cross-reactivity with ratANF, no cross-reactivity with arginine-vasopressine). From saturation experiments [incubation of platelets with increasing concentrations (3.5 to 63.7 pM) of labeled hANF and with (10(-8) M) or without unlabeled hANF], we determined (n = 11): Bmax (m +/- SEM) = 4.5 +/- 0.7 pM or 5.4 +/- 0.8 sites per cell and Kd (m +/- SEM) = 10.84 +/- 1.70 pM.