Detailed immunologic studies were done on 29 patients with colorectal cancer. The plasma level of circulating carcinoembryonic antigen, the in vitro reactivity of the peripheral blood leukocytes (PBL) to colorectal-tumor-associated antigens (CTAA), and the competence of the T cell population were determined. The in vitro reactivity of the PBL to CTAA was determined by a lymphoblastic response and leukocyte migration inhibition. The competent T cell population was determined by enumerating the T and B cells, the rosette-inhibiting titer of antithymocyte globulin, and the reactivity to skin test antigens. An arbitrary score ranging from 0 (low immunocompetence) to 100 (high) was assigned to the results of each test. The mean score or the immunocompetence quotient (ICQ) which ranged from 22 to 100 was judged to reflect the immunocompetence. The sequential ICQ of individual patients strongly suggested that this information reflected the immunocompetence of patients with cancer of the colon.