The origin and physiological functions of the connective tissue mast cells have been the source of considerable speculation and experimentation since they were first described by Ehrlich. In recent years many workers have helped to clarify a few of the biochemical potentialities of these cells.3,14,17,19,20 The role of mast cells in wound healing as well as the production of connective tissue ground substance via the manufacture of sulphated mucopolysaccharides still remains in doubt. Asboe-Hansen1 has noted that the number of mast cells varies, in general, with the quantity of metachromatic ground substance present. Specific examples cited include myxedematous skin and the synovial membrane. However, if mast cells do contribute to the formation of ground substance, the property is not exclusively theirs. No mast cells appear in regenerating rat tendon25; they are similarly absent in the aortic media, which is rich in metachromatic material.5 In studies of