A new mannose-binding lectin, designated SGM2, was isolated from the rhizome of a Chinese medicinal herb Smilax glabra (also known as sarsaparilla in general) by saline extraction, ammonium sulfate precipitation and fractionation, and affinity chromatography on fetuin- and mannose-agarose. SGM2 is shown to have a molecular mass of 37 kDa on gel filtration and 12.5 kDa on SDS-PAGE, indicating that it is a trimeric protein composed of three identical subunits. When the first 30 amino acid residues at the N-terminal were compared, SGM2 had approximately 40% homology with those of some other monocots. SGM2 had the property of hemagglutinating activity toward rabbit erythrocytes, which could be reversed by mannose and mannose polymers. SGM2 exhibited antiviral activities against both herpes simplex virus type 1 (HSV-1) and respiratory syncytial virus (RSV) with the same EC(50) of 8.1 microM.