Immunohistochemical study with various monoclonal antibodies against the mononuclear cell surface antigen was performed on the regional lymph nodes of gastric cancer through Avidin-Biotin Complex (ABC) method. In the paracortical area (P.C.) of those lymph nodes without the metastasis of gastric cancer, T cells were dominant, and OKT3 positive (OKT3+) cells and OKT4+ cells were diffusely present, while OKT8+ cells were occasionally recognized. In the sinus, subsets of the above T cells, and OKT9+ cells and OKT10+ cells were observed. In the germinal center (G.C.), mantle zone (M.Z.) and primary follicle (P.F.), which were B cell regions, OKIa1+ cells and Leu12+ cells were diffusely present. OKB7+ cells, OKT9+ cells, OKT10+ cells and Leu7+ cells were also noted in G.C.. Leu8+ cells were observed in M.Z. and P.F.. OKIa1+ cells were occasionally noted in P.C. and sinus. In the lymph nodes with the metastasis, decrease of OKT4+ cells and increase of OKT8+ cells were noted in comparison to the lymph nodes without the metastasis. Using the tissue double fluorescence staining method, it was found that about half of the OKT4+ cells were helper T cells. The majority of OKT8+ cells were identified as cytotoxic T cells or their precursors. By preoperative endoscopic administration of OK-432 or PSK into the tumor, the IL-2 receptor+ cells, or OKM1+ cells and OKT4+ cells increased in the regional lymph nodes and the antitumor activity was intensified.