The intensity of lymphoid cell infiltration and distribution of lymphocyte subsets in tumors were investigated immunohistochemically on tumor tissues obtained from 11 patients with gastric carcinoma, who had been treated with mitomycin C (MMC), 12 mg/m2, i.v. 5 days before operation. The results were compared with those obtained from 24 untreated patients as controls. In the tumor tissues from pretreated patients, the intensity of lymphoid infiltration was not significantly different from that of untreated patients. However, high-grade infiltration of CD4+ cells was observed in 55% of pretreated patients, whereas only 8% of control patients exhibited the high-grade infiltration (P < 0.02). Since the CD8+ cell infiltration was not significantly altered, the ratio of CD4+ to CD8+ cells was more frequently estimated to be more than 1 in patients pretreated with MMC, as compared to untreated controls (P < 0.02). Further, CD25+ cells in pretreated tumor tissues were more predominant than those in control tumor tissues (P < 0.05). These results suggest that MMC administration induces these alterations in lymphocyte subsets in tumor tissue in patients with gastric carcinoma.