Soluble beta-galactoside binding lectins were prepared from the rat small intestinal mucosa by chromatography on asialofetuin-Sepharose. The lectin fraction exhibits 3 bands with Mr of 21,5 kDa, 19 kDa and 17 kDa on SDS-PAGE. This fraction inhibits a partially purified soluble alpha(1-2)-fucosyltransferase by interaction with the glycoprotein substrate asialofetuin, whereas the inhibition is non competitive for the donor GDP-fucose. It has no effect on other enzymes of the fucosylation system, namely glycosyl-nucleotide pyrophosphatase and the system synthesizing GDP-fucose from GDP-mannose. A different and specific soluble protein inhibitor of fucosyltransferase activity inhibits this activity by a competitive mechanism for GDP-fucose and a non competitive one for asialofetuin. Unlike the lectins, this inhibitor also inhibits the action of pyrophosphatase and the formation of GDP-fucose by different mechanisms. The possible extension of these in vitro results to the in vivo regulation of glycosylation is discussed.