A series of substituted amidinoindoles have been prepared as mimics of the RGD sequence and were studied as antagonists of the platelet glycoprotein IIb-IIIa receptor (GPIIb-IIIa). The agents were potent and selective antagonists of GPIIb-IIIa. Compared to their acyclic counterparts, the amidinoindole series bound with 10- to 20-fold greater affinity, indicating the advantages of added conformational restriction and/or hydrophobicity in the basic region of RGD mimics.