The distribution, density and activation of Langerhans cells (LC) has been established in biopsies of normal human buccal mucosa, hard palate, lateral border and dorsum of tongue, floor of mouth and lip taken from sudden death post mortems. LC were identified in cryostat sections with monoclonal antibodies against CD1, HLADR, HLADQ and HLADP using an immunoalkaline phosphatase technique. The use of post mortem material was validated by comparison with biopsies taken from volunteers. LC were predominantly situated in the basal and immediately suprabasal layers of the epithelium. In floor of mouth, lip, lateral border and dorsum of tongue the cells were found along the length of the epithelium. In buccal mucosa, although LC showed fundamentally a similar distribution, a tendency to cluster around the connective tissue papillae was also noted. In hard palate LC were found parallel to the surface in the mid zone of the epithelium. No evidence of LC free areas was found. Dorsum of tongue had the highest density of LC per mm epithelial surface length (28.3 cells per mm) which was significantly greater (P less than 0.05) than buccal mucosa (25.2) which in turn had significantly more cells (P less than 0.05) than lip (22.4). The lowest density of LC was found in lateral border of tongue (17.6), hard palate (17.6) and floor of mouth (16.7). These sites had significantly fewer cells than lip, buccal mucosa and dorsum of tongue (P less than 0.05). Class II MHC molecules are necessary for antigen presentation and in all sites except buccal mucosa there was no significant difference between the number of cells expressing CD1, HLADR, HLADQ and HLADP.(ABSTRACT TRUNCATED AT 250 WORDS)