The quantitative distribution of immunoglobulin-(Ig)-producing cells of the major classes was determined by paired immunohistochemistry in the proximal jejunal mucosa of 29 patients with dermatitis herpetiformis (DH). The specimens were categorized stereomicroscopically in three groups. Compared with controls, the total number of Ig-producing cells in a defined "mucosal tissue unit" was for group I (normal or minor abnormality) found to be raised by a factor of 1.4, but the class ratios remained normal. In group II (major abnormality) the total immunocyte number was raised 2.6 times; IgA, IgM, and IgG cells showed a 2.4-, 3.5-, and 5.1-fold increase, respectively. Group II (intermediate abnormality) fell between the two other groups with regard to changes in the immunocyte population. Altogether the local immunocyte pattern was very similar to that previously found in coeliac disease (CD) specimens of comparable mucosal abnormalities. IgD cells were few, and showed no increase compared with controls. Also IgE cells were few, but were encountered more frequently than in controls. As in CD, staining for extracellular Ig in the lamina propria tended to be more intense than in controls. Normal or increased staining for SC, IgA and IgM in the crypt epithelium indicated a normal external transfer of these components. DH differed from DC in showing no significant rise in the level of serum IgA. Serum IgM was reduced in group III, and IgG was increased in group I.