The distribution and activation of T-lymphocyte subsets in the small intestinal mucosa of coeliac disease and dermatitis herpetiformis subjects on a normal diet has been studied and compared to normal controls. Double-labelling immunofluorescence techniques with monoclonal antibodies were used on cryostat tissue sections. Intestinal epithelial cells demonstrated staining for HLA-DR, the intensity being proportional to the degree of enteropathy. In both patients and controls nearly all (97%) intra-epithelial lymphocytes were of the CD8 subset and not activated as judged by HLA-DR expression. In the lamina propria there was an approximate 50-fold increase in T cells in the patients as compared with the controls. Whilst the ratio of total CD4 to total CD8 cells was unchanged, the CD4 subset was preferentially activated in the patients. Thus in the normal controls the median ratio of activated CD4 cells to activated CD8 cells was 1.67 whilst for dermatitis herpetiformis and coeliac disease it was 3.42 and 6.07 respectively. These findings suggest that the lamina propria is a site of vigorous T-cell activity in gluten-sensitive individuals and is consistent with the view that the enteropathy of dermatitis herpetiformis and coeliac disease is the result of a delayed-type hypersensitivity against gliadin.