Lipase modified with polyethylene glycol became soluble and active in organic solvents, and catalyzed regioselective deacetylation of peracetylated monosaccharide derivatives in 1,1,1-trichloroethane. The deacetylation occurred only at the positions of C-4 and C-6 of the glycopyranoside ring. Especially, peracetylated methyl beta-D-xylopyranoside and peracetylated L-serine-beta-D-xylopyranoside were hydrolyzed only at the position of C-4. Subsequently, one of the resulting products, that is L-serine-2,3-di-O-acetyl-beta-D-xylopyranoside, was coupled with galactose residue to obtain L-serine-4-O-(beta-D-galactopyranosyl)-beta-D-xylopyranoside, a model compound of the carbohydrate-protein linkage region of proteoglycans.