Analysis of epidermal growth factor (125J-EGF) binding to human red cells revealed the presence of two classes of binding sites with apparent equilibrium dissociation constants (app.Kd) in the 10(-10)-10(-9) M and in the 10(-8) M range, respectively. The number of binding sites/cell ranged between 600 and 2,400 for the high-affinity binding site and between 7,200 and 23,000 for the low-affinity site. No differences were seen in the apparent Kd values for both types of binding sites between red cells obtained from donors with different ABO-blood groups. An increase in the number of high affinity EGF binding sites was observed in donors with blood group A1-erythrocytes as compared to red cells taken from donors with blood groups O and B.