A monoclonal antibody (MoAb) (LYP18), generated against human platelet glycoprotein IIb/IIIa (GPIIb/IIIa), immuno-precipitated a IIb/IIIa-like GP complex from a highly tumorigenic human melanoma cell line (M3Dau). The M3Dau melanoma cells specifically bound 125I-labeled LYP18. To study the biologic role of these IIb/IIIa-like glycoproteins, M3Dau melanoma cells were incubated with LYP18 or a control MoAb directed against another melanoma cell-surface antigen and implanted subcutaneously (SC) in nude mice. LYP18 dramatically inhibited the growth of tumor in vivo. LYP18 was not directly cytotoxic to the melanoma cells. These results demonstrate that the IIb/IIIa-like GPs are present on melanoma cells and play a crucial role in tumor cell growth. MoAbs directed against tumor cytoadhesive receptors may represent a novel approach in tumor treatment.