The level of 6-keto-PGF_1α and thromboxane B₂ (TXB₂) in plasma was determined with radioimmunoassay in 58 normal subjects and 92 patients with various cancers (including lung, hepatic, gastric, esophageal and pancreatic carcinoma). The results showed that 6-keto-PGF_1α in plasma was 10.21±2.75 Pg/ml, and TXB₂ 146.03±37.31 Pg/ml in normal individuals, the ratio of 6-keto-PGF_1α to TXB₂ was 0.07; while in cancer patients 6-keto-PGF_1α was 27.5±16.9 Pg/ml and TXB₂ 315.4±173.4 Pg/ml, the ratio of 6-keto-PGF_1α to TXB₂ was 0.08. The values of 6-keto-PGF_1α and TXB₂ in plasma of cancer patients were 2.69 folds and 2.16 folds higher than that of the two groups, respectively. The difference between the two groups was statistically significant (P<0.01). It indicates that, the synthesis and release of PGI₂ and TXA₂ of cancer tissues increases greatly as compared to the normats.