Galectin-3 is a beta-galactoside binding protein expressed by activated macrophages, epithelial cells, and certain other cell types. Galectin-3 has a C-terminal carbohydrate binding domain, an N-terminal part consisting of a proline- and glycine-rich repetitive domain, and a small N-terminal domain. Two independent LPS binding sites on galectin-3 were demonstrated by binding of biotinylated LPS to immobilized recombinant galectin-3. One appears to be the carbohydrate binding site in the C-terminal domain that confers binding of LPS from Klebsiella pneumoniae that has a p-galactoside-containing polysaccharide chain, This binding is best demonstrated using galectin-3 immunocaptured by a mAb to the N-terminal part (M3/38) and is inhibited by lactose. In contrast, Salmonella minnesota R7 LPS (Rd mutant), which is devoid of beta-galactosides, appears to bind to a site within the N-terminal part of galectin-3. This interaction is best demonstrated using galectin-3 directly immobilized in wells, and it is inhibited by the Ab M3/38, but not by lactose. Binding inhibition by polymyxin B and the profile of inhibition by a panel of LPSs with different amounts of the inner and outer cores present indicate that this second binding site recognizes the lipid A/inner core region of LPSs.