Pectin is one of the major cell wall polysaccharides found in dicotyledonous plants. We have solubilized and partially purified a beta-(1-->4)-galactosyltransferase (GalT) involved in the synthesis of the beta-(1-->4)-galactan side chains of pectin. The enzyme protein was almost completely solubilized by mixing a crude microsomal preparation of etiolated 6-day-old soybean (Glycine max Merr.) hypocotyls with a detergent, Triton X-100 (0.75%, w/v), in buffer. The solubilized enzyme was partially purified by ion-exchange chromatography. The crude membrane-bound GalT transferred Gal from UDP-Gal onto 2-aminobenzamide (AB)-derivatized beta-(1-->4)-galactoheptaose (Gal(7)-AB), leading to the formation of Gal(8-11)-AB by attachment of a series of one to four galactosyl residues; this is similar to what has previously been observed for 2-aminopyridine-derivatized beta-(1-->4)-galactooligomer acceptors (Konishi et al. in Planta 218:833-842, 2004). The partially purified GalT, by contrast, was able to transfer more than 25 galactosyl residues and elongated the chains to about Gal(35)-AB, thus almost reaching the length (43-47 Gal units) of native beta-(1-->4)-galactan side chains found in pectic polysaccharides from soybean cotyledons (Nakamura et al. in Biosci Biotechnol Biochem 66:1301-1313, 2002). Enzyme activity increased with increasing chain length of beta-(1-->4)-galactooligomers and reached maximal activity at heptaose, whereas galactooligomers higher than heptaose showed lower acceptor efficiency.