The correlation among lymphoid infiltration (LI) in the primary tumor, cell-mediated immunity and the accumulative 5-year survival rate was studied in 515 patients with resected gastric cancer. As scales of cell-mediated immunity, the blastformation rate of peripheral lymphocytes against phytohemagglutinin (PHA)-M (Difco) and the PPD skin reaction were examined. The results were as follows: 1) LI of the primary tumor was found in 124 out of 515 patients (24.1%); patients with marked LI were 6.4%. 2) The blastformation rate in patients with positive LI was significantly higher than that in patients with negative LI (p<0.01). 3) The LI of the primary tumor tended to be found commonly in patients with gastric cancer in clinical stage I. 4) The blastformation rate decreased when the clinical stage of gastric cancer progressed, however, this trend was not seen in patients with positive LI regardless of the stage. 5) The accumulative 5-year survival rate in patients with positive LI was longer than that in the patients with LI negative (p<0.05). 6) There was no relationship among the degrees of LI in the primary tumor, the PPD skin reaction and lymphocyte count. In conclusion, LI of the primary tumor has a close relationship with the blastformation rate, and this phenomenon suggests that a local appearance of cell-mediated immunity in the cancer bearing body may occur.