Chemical and histochemical analyses, including testicular and staphylococcal hyaluronidase digestion, have been made of a jaw myxoma and the results show the presence of two acid mucopolysaccharides. Of the total mucopolysaccharide present 80% was hyaluronic acid and 20% chondroitin sulphate. The high content of non-sulphated mucopolysaccharide would seem to explain the paucity of fibres characteristic of the myxoma. It is suggested that myxomas generally probably have a similar high hyaluronic acid content. It is considered that the cell concerned is a mesenchymal cell elaborating non-sulphated mucopolysaccharide and may be called a ;myxoblast'; it is metabolically different from the sulphated-mucopolysaccharide-collagen-producing fibroblast. The high hyaluronic acid content is much greater than that found in embryonic connective tissue and may be a significant factor in the neoplastic behaviour of the myxomatous tissue. The aggressive behaviour of the myxoma is against a simple reversion to embryonic mesenchyme. It is concluded that the myxoblast is an active mucopolysaccharide-secreting cell and that mucin in the myxoma is not a sign of cell degeneration of preexisting fibroblasts or collagen.