Thrombin is a serine protease able to evoke biological responses from a variety of cells, including platelets, endothelial cells, fibroblasts and smooth muscle cells. The structure of the thrombin receptor present in the human megakaryoblastic cell line and in hamster fibroblasts has recently been deduced by expression in the Xenopus laevis oocyte. The cloned receptor is a new member of the seven transmembrane domain receptor family that interacts with G proteins. A large amino-terminal extracellular extension has a cleavage site for thrombin (Leu Asp Pro Arg/Ser Phe Leu Leu,/representing the cleavage site). Thrombin cleaves at this site, unmasking a new amino terminus, that functions like a ligand, binding to an as yet undefined site and eliciting receptor activation. Peptides similar to a new amino terminus created after cleavage are able to mimic thrombin cellular effects. These agonist peptides are used to analyse the role of the cloned receptor in the thrombin-specific response.