One hundred large bowel carcinomas were studied immunohistochemically with regard to expression of HLA-DR antigen (DR). One or two sections from each tumor including surrounding normal mucosa were examined by a semiquantitative counting system for tumor cells and mucosal and stromal infiltrates of lymphocytes and mononuclear cells (MNCs) with DR expression and the results were applied Chi-square test. The rate of presence of DR positive (DR+) lymphocytes in lymphoid nodules and DR+ lymphocytes/ MNC in the adjacent mucosa and stroma in DR+ carcinoma (50%) was higher (P < 0.01) than in DR- carcinoma (21.9%). Thirty-six carcinomas (36%) were DR+. Three (75%) out of 4 DR+ poorly differentiated carcinomas and six (20%) out of 30 DR+ moderately differentiated carcinomas showed homogeneously strong DR+ expression. There was tendency for poorly differentiated carcinoma to be more homogeneous DR+ expression. According to Dukes' stage, four (80%) out of 5 carcinomas in Dukes' stage D were DR-. An increased infiltration of lymphocytes/MNCs into adjacent mucosa and stroma in large bowel carcinomas is possibly related with DR expression by carcinoma. From the results of this study, we postulated as follows: 1) DR+ tumor cells may act as antigen-presenting cells, 2) They may have an inhibitory effect for distant metastasis, 3) Poorly differentiated carcinoma expressed more DR+ homogeneously.