The site of the rheumatic lesion is in the skeletal tissues of mesenchymal origin, viz., connective tissue, fascia, muscle, ligament, joint capsule, synovial membrane, articular cartilage, and even bone. These all contain collagen. The basic pathological lesions of rheumatism also occur in blood vessels which supply these tissues. The basic lesion is fibrinoid degeneration of the collagen bundles of mature fibrous tissue. Chondroitin sulfuric acid and hyaluronic acid are constituents of the ground substance in which collagen fibrils are embedded. Hyaluronic acid is a simple viscous mucopolysaccharide. It forms a gell with proteins but is not an-tigenic. The enzyme hyaluronidase, which breaks down hyaluronic acid, is found in type II pneumococci, A and C hemolytic streptococci, staphylococci and many anaerobes. It is also present in leeches, snake venom, the testis. There is suggestion that when hyaluronidase attacks hyaluronic acid and other ground substances of connective tissue there occurs a series of changes analogous to those seen in rheumatic diseases. These researches on hyaluronic acid and hyaluronidase reveal the wider recognition of the fact that we must have a deeper understanding of the fundamental problems of connective tissue metabolism if we are to have a clear knowledge of "rheumatism.".