Elastin-like polypeptides (ELPs) are artificial polypeptides with unique properties that make them attractive as a biomaterial for tissue-engineered cartilage repair. ELPs are composed of a pentapeptide repeat, Val-Pro-Gly-Xaa-Gly (Xaa is any amino acid except Pro), that undergo an inverse temperature phase transition. They are soluble in aqueous solution below their transition temperature (T-t) but aggregate when the solution temperature is raised above their T-t. This study investigates the theological behavior of an un-cross-linked ELP, below and above its T-t and also examines the ability of ELP to promote chondrogenesis in vitro. A thermally responsive ELP with a T-t of 35 degreesC was synthesized using recombinant DNA techniques. The complex shear modulus of the ELP increased by 3 orders of magnitude as it underwent its inverse temperature phase transition, forming a coacervate. or gel-like, ELP phase. Values for the complex shear moduli of the un-cross-linked ELP coacervate are comparable to those reported previously for collagen, hyaluronan, and cross-linked synthetic hydrogels. Cell culture studies show that chondrocytes cultured in ELP coacervate maintain a rounded morphology and their chondrocytic phenotype, characterized by the synthesis of a significant amount of extracellular matrix composed of sulfated glycosaminoglycans and collagen. These results suggest that ELPs demonstrate great potential for use as in situ forming scaffolds for cartilaginous tissue repair.