It is well established that after metastatic cancer cells escape the primary tumor and enter the circulation, their interactions with microvascular endothelium of a target organ constitute an essential rate-limiting step in hematogenous cancer metastasis. However, the physiological and biochemical processes supporting neoplastic cell arrest and retention in the microcirculation are still poorly understood. In this study, we present experimental evidence that microvascular endothelium of metastasis-prone tissues undergoes activation in response to desialylated cancer- associated carbohydrate structures such as Thomsen- Friedenreich (TF) antigen (Gal1-3GalNAc) expressed on circulating glycoproteins and neoplastic cells. The metastasis-associated endothelium activation, manifested by marked increase in endothelial cell surface galectin-3 expression, causes gradual decrease in cancer cell velocities (from 72· 10² ±+ 33 ·; 10 ² & [mu]m s ^-1 to 7.6·; 10 ² & [plusmn]+ 1.9 ·; 10 ² µm s ^{- 1}, mean ±+ S.D.) accompanied by a corresponding increase in the percent of rolling cells (from 3.3% ±+ 1.2% to 24.3% ±+ 3.6%, mean ±+ S.D.), and results in human breast and prostate carcinoma cell arrest and retention in the microvasculature. This process, which could be of high importance in hematogenous cancer metastasis, was inhibited efficiently by an anti-TF antigen function- blocking antibody. Carbohydrate-mediated endothelial activation could be a process of physiological significance as it likely occurs in the interactions between a variety of circulating constituents and the vessel wall.