The synovial membrane-articular cartilage junction has been studied with electron microscopy in 20 patients with rheumatoid arthritis. Material came from specimens taken at synovectomy operations in the majority in an attempt to avoid far advanced disease. Comparisons were made with osteoarthritic tissue and with one normal control case (a meniscectomy). The process of cartilage destruction in RA appears to be multifactorial in origin. The pannus showed to distinct appearances being either cellular and usually vascular, or more fibrous in appearance. These may be two phases of the one process. There was evidence of collagenase activity in junctional cells and for deeper chondrocytes playing a role of polymorphs, a cell given considerable emphasis for cartilage destruction from biochemical studies. Nutritional factors may also be involved and the invasion of rheumatoid granulation tissue may be provoked by chemotaxis from immune complexes in the cartilage surface. The place of a substance similar to tumour angiogenic factor remains uncertain from morphological evidence. The separate phases of the reaction between synovium and cartilage means that responses to various anti-rheumatic drugs may vary widely, and this fact should be appreciated by those testing drugs experimentally.