The competitive inhibition curves of 125I-echistatin bound to U87MG glioma cells in the presence of increasing concentrations of cyclic RGD peptide. Their IC50 values were calculated to be 47±2, 35±2, 37±2, 85±1 and 422±15 nM for K(HYNIC)2-2P-RGD2, K(HYNIC)2-3P-RGD2, K(HYNIC)2-3G-RGD2, K(HYNIC)2-RGD2 and c(RGDyK), respectively. The integrin αvβ3 binding affinity follows the order: K(HYNIC)2-3G-RGD2 ~ K(HYNIC)2-2P-RGD2 ~ K(HYNIC)2-3P-RGD2 > K(HYNIC)2-RGD2 ≫ c(RGDfK).