In order to assess the effects of malignant tumors on the immune system, 25 spleens from patients with gastric carcinoma were studied by in situ immunohistochemical methods for lymphocyte subsets and cells of the mononuclear phagocyte system. Highly significant reductions of CD4+ T cells (P less than 0.001), Ki M2+ and Ki M-3+ MPS cells (P less than 0.02 and P less than 0.05), and a stage-dependent reduction of Ki 67+ B cell proliferation activity (P less than 0.05) were seen in spleens of patients with gastric cancer. These results, which were obtained by morphologic methods in a noninvolved lymphatic organ, reflect the systemic immunosuppressive and immunodepleting effects of malignant tumors that are probably mediated by tumor-associated cytokines.