Epithelial dendritic cells (EDC) were examined in human oral tissues with non‐specific keratosis, lichen planus and squamous cell carcinoma. Acetone‐fixed frozen sections were stained using an indirect immunoperoxidase technique and monoclonal antibodies to the human CD1 thymocyte (OKT6) and HLA‐DR antigens. Significantly more T6+and DR+EDC were present in lichen planus tissues than normal controls, tissues with non–specific keratosis and the epithelia overlying/adjacent to squamous cell carcinomas, the latter tissues having comparable numbers of both T6+and DR+EDC. By contrast, significantly fewer T6+EDC and significantly more DR+cells were present in the invasive epithelium of squamous cell carcinomas than the overlying/adjacent epithelium of carcinomas, the non‐specific keratosis group and the normal tissues. 23–60% of pathological tissues had either focal or general DR+reactivity in keratinocytes, but there was no correlation between the density of T6+or DR+EDC and the keratinocyte DR status of the tissues. The results suggest that immunological enhancement occurs in lichen planus and possibly immunological impairment may characterize invasive squamous cel