Studies were carried out on the in vivo staining properties of fifty patients with proved or suspected oral neoplasia. Toluidine blue, a nuclear stain, was employed, and acetic acid 1 per cent was used for decolorization. Normal oral mucosa, with certain exceptions, does not stain. Our studies indicate that in vivo staining with toluidine blue is useful for: (1) the detection of areas of carcinoma in situ; (2) the recognition of small, early invasive carcinoma; (3) delineation of the margins of larger epithelial neoplasms; (4) the recognition of postsurgical marginal recurrence; (5) The recognition of recurrence or new lesions postirradiation; and (6) the delineation of areas of field cancerization.