Large amounts (2.3 mg) of an inactive form of the glycoprotein GPIIb-IIIa were obtained in highly purified form from 7-liter cultures of human erythroleukemia (HEL) cells. The purified GPIIb-IIIa was converted to an activated form after its immobilization to 96-well plastic plates. The binding of fibrinogen to the activated GPIIb-IIIa was investigated using 24 kinds of RGD peptides and showed that the IC50 values of these peptides correlated with those obtained with activated platelets. These findings indicated that the binding properties of the activated GPIIb-IIIa from HEL cells are quite similar to those of the platelets.