We attempted to clarify the accumulation of radiolabeled lymphocytes to tumors and regional lymph nodes in patients with gastric carcinoma. The effects of oral administration of OK-432 were also studied. Five patients with gastric cancer and one who underwent endoscopic mucosal resection, were entered in the study. Prior to the tracer study in 3 patients with gastric cancer, 5 KE of OK-432 was administered for 2 days. Peripheral mononuclear cells were separated and labeled with [111] In-tropolone. After the resection of stomach, tumor tissue, normal gastric mucosa, regional lymph nodes, and non-regional lymph nodes were dissected and radioactivity was measured by a gamma-counter. Accumulation of lymphocytes to the tumor tissue and n1 lymph node station was more than two times that in the normal gastric mucosa and ten times that in non-regional lymph nodes. Accumulation of lymphocytes to the n2 station was strongly enhanced by oral administration of OK-432.