Fibrinogen-platelet interaction was studied in suspensions of platelets obtained from patients with uncontrolled diabetes mellitus of long duration and from control individuals. Fibrinogen binding sites were exposed by stimulating platelets with ADP or with chymotrypsin. There was no significant difference in fibrinogen mediated aggregation between ADP-stimulated platelets of 80 control and 47 diabetic subjects. The Km values for fibrinogen mediated aggregation of ADP-stimulated platelets obtained from control and diabetic donors were 1.39 +/- 0.13 X 10(-7)M and 1.44 +/- 0.13 X 10(-7)M; the Vmax values (expressed in arbitrary light transmission units) were 87.8 +/- 3.14 and 92.8 +/- 4.5 (mean +/- S.E.M.). The analysis of variance showed no significant relationship between Km, Vmax, age and sex in control group; in patient group there was no significant relationship between Km, Vmax, age, sex, type of diabetes, presence of vascular complications and type of treatment (insulin and/or oral hypoglycemic agents). Fibrinogen mediated aggregation of chymotrypsin-treated platelets showed similar pattern in 25 control and in 25 diabetic donors. In 24 normal individuals and in 24 diabetic patients Scatchard analysis revealed 48,820 +/- 5350 fibrinogen binding sites per one normal platelet (Kd = 4.7 X 10(-7)M) and 45,350 +/- 4663 sites per one diabetic platelet (Kd = 3.5 X 10(-7)M). Our data suggest a normal pattern of interaction between fibrinogen and fully activated platelets of diabetic subjects.