Fibrinogen binds to specific receptors on human washed platelets and these sites are induced by adenosine diphosphate (ADP). This interaction is assumed to be the basis for the participation of the molecule in ADP-stimulated aggregation of platelets, but fibrinogen binding to platelets in plasma has not been directly demonstrated. In this study, we have characterized the interaction of 125I-fibrinogen to platelets in the platelet-rich plasma (PRP) of afibrinogenemic patients. In either citrated or heparinized PRP, association of fibrinogen with platelets was demonstrable and was dependent on ADP dose. This binding reached equilibrium in 10–15 min, and saturation was achieved at fibrinogen concentrations greater than 0.5 microM. A linear Scatchard plot was derived that indicated a single class of binding sites with an affinity constant of Ka = 1.8 X 10(6) M(-1), and 32,000 fibrinogen molecules were maximally bound per platelet. The kinetics of the platelet fibrinogen interaction in plasma were essentially the same at 37 degrees C and 22 degrees C, but fewer molecules were bound at 37 degrees C. The rate constants of association were k 22 degrees C on = 0.9 X 10(6) M-1 . min-1 and k 37 degrees C on = 0.4 X 10(6) M-1 . min- 1, respectively. Stabilization of the platelet-bound fibrinogen occurred only to a partial extent in both heparinized and citrated plasma. These results are similar to those obtained with washed platelets and establish that the previously defined steps in ADP- induced binding of fibrinogen to platelets occur in plasma, namely receptor induction by ADP, initial reversible binding, and irreversible binding.