44 patients with large bowel cancer were randomly divided into two groups, therapeutic and control group. The level of serum selenium, T lymphocyte subsets consisted of CD3, CD4, CD8, CD4/ CD8, NK and LAK cell activity were measured preoperation and postoperation. Simultaneously se content in tumor and normal tissue of the large bowel were measured in 35 cases. Serum se level (0.81 +/- 0.14 umol/L) was lowered in patients with large bowel cancer and increased significantly after se supplementation in the therapeutic group (P < 0.01). It was significantly different from that of the control group (P < 0.01), CD3, CD4, CD4/CD8, NK and LAK cell activity were obviously increased postoperatively in the therapeutic group and significantly different from those of the control group. The results suggest that supplement of Se can promote cell-mediated immunity in humans. In addition, Se can promote cell-mediated immunity in humans. The Se level of 22. 13 +/- 1.76 umol/g in tumor was significantly lower than that of 24.30 +/- 1.96 umol/g in normalmucosa in case of large bowel cancer (P < 0.01). This indicates that there may be a close relationship between low se level and the carcinogenesis of the colon and rectum.