Samples (110) of human mandibular gingiva and buccal mucosa, harvested from patients undergoing third-molar surgery, were subjected to in vitro labelling with tritiated thymidine, bromodeoxyuridine, or a sequential double-labelling technique comprising tritiated thymidine followed by bromodeoxyuridine, in order to determine the efficacy of a new incubation labelling technique, and to characterize S-phase labelling indices in human oral mucosa. Whilst, there were no demonstrable differences in labelling indices obtained by single thymidine, single bromodeoxyuridine or double labelling, there was a significant difference between anatomical sites, with higher S phase labelling observed in buccal mucosa (mean LI 11.7) than mandibular gingiva (mean LI 8.5; P<0.01). There was, however, no significant correlation between individual labelling indices and patient age, sex or the time of day when tissue was harvested. The in vitro labelling technique provides a reliable and quantifiable method of characterizing proliferative labelling indices in the human oral cavity. Further investigation is being carried out to profile wider age and anatomical ranges and to utilize the double-labelling technique to calculate S-phase durations and cell-cycle times. These profiles may have a future role in the assessment of oral mucous membrane disease.