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The subsets of tumor-infiltrating lymphocytes (TIL) and prostaglandin (PG) E2were measured in the resected tissues of 32 colorectal cancers without metastasis and 14 with metastasis in order to investigate the local immunity in metastasis of colorectal carcinoma. Subsets of TIL (Leu 1, Leu 2a, Leu 3a, Leu 10, Leu 1 1b, IL-2 receptor) were detected by immunohistochemical staining of frozen tissues. The number of positive cells was counted and expressed as number positive per 250 × 250μm2. The numbers of T cells (Leu 1) and natural killer cells (Leu 11b) were larger in early cancers and decreased in parallel with the presence of metastasis (control [n=9]: 89±28, 6±4; early cancers [n=9]: 269±112*, 76±56*;advanced cancers without metastasis [n=11]: 182±80*, 56±59*;advanced cancers with metastasis [n=11]: 76±42*, 26±21; values are mean ± SD;*P< 0.05, ANOVA). The level of PG E2from the draining vein (V) measured by radioimmunoassay was higher than that from the feeding artery (A) (119.1±14.3vs.15.4±1.9 pg/ml;P<0.001). The PG E2V/A ratio of cancers with metastasis was significantly higher than that of those without metastasis (132±2.4vs.5.6 ±0.8;P<0.001). TIL was decreased in parallel with the increase of PG E2V/A ratio. We conclude that TIL and PG E2may play an important role in metastasis of colorectal carcinoma and that PG E2has an adverse effect in suppressing local immunity and enhancing metastasis.