Lymphocyte subpopulations, plasma cells in the lamina propria, and the expression of HLA-DR antigens on the epithelium of the rectal and colonic mucosae were studied in eight children with ulcerative colitis and 12 control subjects using a panel of monoclonal antisera and the peroxidase technique before any treatment and 3 months later (four patients). The numbers of intraepithelial lymphocytes were similar in specimens from patients and controls. The majority of these cells (on average, 73% in the patients and 84% in the controls) were mature CD3+ T cells; among them, CD8+ suppressor-cytotoxic cells were preponderant. In both untreated and treated patients, the numbers of mature T cells in the rectal mucosae were supranormal (1,870 +/- 205 cells/mm2, p less than 0.01 and 1,537 +/- 214 cells/mm2, p less than 0.05, respectively; controls, 1,105 +/- 214 cells/mm2). In rectal specimens from untreated patients, the number of helper T (CD4+) cells was increased (1,094 +/- 74 versus 801 +/- 74 cells/mm2, p less than 0.05); the same specimens had more B-1-positive (CD20+) B cells and pre-B cells (122 +/- 21 versus 71 +/- 17 cells/mm2, p less than 0.05). The number of IgG-containing cells was significantly greater than in the controls (1,058 +/- 263 versus 359 +/- 64 cells/mm2, p less than 0.01), and the commonest isotype in the plasma cells of patients was IgG. The number of IgE-containing cells was also significantly elevated (230 +/- 40 versus 95 +/- 16 cells/mm2, p less than 0.01). The numbers of IgA- and IgM-containing cells were similar in patients and controls.(ABSTRACT TRUNCATED AT 250 WORDS)