Nineteen pieces of muco-gingival tissue from man and monkey were examined by histological and histochemical methods. Neither a surface groove nor a change from keratinisation to non-keratinisation were consistently seen in all specimens; the differing elastic tissue content of the gingiva and alveolar mucosa was therefore used to define the muco-gingival junction. The surface groove, the keratinisation pattern, the distribution of hydrolytic enzymes and the glycogen content have been related to this junction. The gingiva and alveolar mucosa meet without a gradual transition zone suggesting that an intrinsic difference exists between them. This junction provides a useful model system for the examination of epithelial-connective tissue interrelationships.