Objective. To investigate the interactions between chondrocytes and their extracellular matrix (ECM). An attachment assay was used to determine the extent of integrin-mediated attachment of chondrocytes to a variety of ECM proteins and the effect of monolayer culturing on attachment activity.Methods. Bovine and human articular cartilage chondrocytes were grown in high-density monolayer cultures for 3-21 days and used in the assays. Cell shape and production of 3H-proline-labeled collagen were monitored to assess phenotypic changes with time in culture. Cultured chondrocytes were incubated in wells coated with purified proteins, with and without specific inhibitors of integrin-mediated attachment, and cell attachment was determined.Results. Compared with bovine serum albumin, chondrocytes showed significant attachment to fibronectin, matrix Gla protein (MGP), osteopontin, bone sialoprotein I1 (BSP 11), vitronectin, and types I1 and VI collagen. A synthetic peptide containing the integrinrecognition sequence Arg-Gly-Asp inhibited attachment to all the proteins tested, except types I1 and VI collagen. A monoclonal antibody (MAb) to the PI-integrin subunit inhibited attachment to fibronectin, MGP, and type I1 collagen, and a MAb to the P,-integrin subunit inhibited attachment to BSP I1 and osteopontin. An increase in cell attachment was seen with time in culture, and this increase was followed by a change in the chondrocytes to flattened, type I collagewproducing cells.Conclusion. Chondrocytes can attach to a variety of cartilage and bone proteins; this attachment is mediated via integrins, including members of both the PI and P3 subunit families. The modulation of the chondrocyte phenotype during monolayer culture may be related to activation or increased expression of integrins.