The galactose-specific lectin present in the seeds of snake gourd (Trichosanthes anguina) was purified in high yield by affinity chromatography on cross-linked guar gum. The purified snake gourd seed lectin (SGSL) yielded a single symmetrical peak on gel filtration with an M(r) of 62 kDa and gave a single band in PAGE under non-denaturing conditions. In SDS-PAGE, SGSL gave a single band of M(r) 53 kDa in the absence of beta-mercaptoethanol, and two bands of M(r) 32 and 23 kDa in its presence, indicating that the lectin is a heterodimer in which the subunits are linked by a disulphide bridge. The lectin gave a single precipitin line in immunodiffusion experiments with antiserum raised against the purified SGSL. No cross-reactivity was found between SGSL and antiserum raised against the Momordica charantia lectin and vice versa, suggesting that the two lectins are antigenically dissimilar. Haemagglutination-inhibition data show that Me beta D-Gal is the best monosaccharide inhibitor of SGSL and indicate that an equatorial hydroxyl at C-2 and axial hydroxyl at C-4 in the pyranose form are important binding loci for the lectin.