Sodium alginate was degraded by alginate lyase from Corynebacterium sp., and the product was purified by an ultrafiltration (UF) membrane module. The UF treatment was carried out at a transmembrane pressure of 0.15 MPa and a flow velocity of 0.6 m/s in the cross-flow mode, and non-degraded alginate was almost completely removed. The alginate oligosaccharide obtained was a mixture of di- to octasaccharides and had promoting activity toward lettuce root elongation (about 2-fold compared with the control) in the concentration range of 200-3000 microg/ml. The effect of the degree of polymerization on this activity was examined by using each oligosaccharide fractionated by gel chromatography. The tri-, tetra-, penta- and hexasaccharides were each found to have root growth-promoting activity in a lettuce bioassay.