Expression of HLA-DR antigens and infiltration of T-lymphocyte subsets (CD4, CD8), cell activation marker (CD25), B cells (CD20), macrophages (CD68 and Ber-Mac 3) and natural killer cells (CD56) in the rectal mucosa of patients with bacillary dysentery and in healthy controls were studied in an effort to interpret the immunopathological changes taking place in the rectal mucosa during the acute phase of shigellosis. The epithelium of the rectal mucosa from 21 of 32 patients was HLA-DR+. Conventional histology showed acute inflammation in 16 of these patients, chronic inflammation in 3, and in 2 histology was normal. In 7 of 20 controls the epithelium was HLA-DR+; 4 of these 7 were found to suffer from chronic inflammation, whilst in 3 the mucosa was normal. The number of HLA-DR+ intraepithelial lymphocytes in biopsies from patients with Shigella infection was significantly higher (p = 0.005) than in controls. The infiltration of CD8+ cells in the surface epithelium and in the lamina propria, and of CD4+ cells in the lamina propria alone, was significantly higher (p< 0.001) in patients than in controls. The results demonstrate that infiltration of T cells with suppressor/cytotoxic or helper/inducer phenotype in the epithelium and in the lamina propria in Shigella-infected patients may be related to the induction of HLA-DR expression in non-lymphoid cells during acute Shigella infection.