The MHC class II subregion products (HLA-DR), HLA-DP, and HLA-DQ) were located by immunofluorescence in serial sections of ethanol-fixed, paraffin-embedded jejunal mucosa from control subjects and patients with coeliac disease (CD). DR staining was seen in a granular luminal distribution and basolaterally on surface epithelial cells in both untreated and treated CD patients and in controls. In untreated CD the crypt epithelium was positive for DR almost to the bottom of the glands. This contrasted with virtually absent glandular DR staining in controls and weak staining including only the upper part of the crypts in 5 out of 11 treated patients. HLA-DP was present apically in the surface epithelium in all untreated patients, in 5 out of 11 treated patients, and in 4 out of 11 controls. HLA-DQ appeared only in three untreated patients and was restricted to patches of surface epithelium. The number of intraepithelial T lymphocytes per millimetre of surface epithelium was significantly higher in untreated than in treated CD patients or controls; it was also significantly higher in specimens with epithelial DP expression than in those without. This suggested that intraepithelial lymphocytes modulate epithelial class II expression.