An enzymatic process to obtain 4-O-.beta.-D-galactopyranosyl-D-xylose useful in compositions or solutions in the in vivo evaluation of intestinal lactose activity in humans, that comprises the steps of preparing a reaction mixture of D-xylose, a .beta.-D-galactopyranoside and a reaction medium that comprises water buffered to a pH between 5.0 and 9.0; adding 10 to 1,000 units of .beta.-D-galactosidase per gram of .beta.-D-galactopyranoside; subjecting the reaction mixture to a reaction or a temperature between a temperature higher than the freezing point of the reaction mixture and 45.degree. C., for 2 to 48 hours; for the reaction by deactivation of the .beta.-D-galactosidase; and to isolate and crystallize the fractions that contain 4-O-.beta.-D-galactopyranosyl-D-xylose from a crystallization mixture selected between mixtures of acetone/methanol in a ratio between 5/1 to 20/1 and mixtures of acetone/water in a ratio between 5/1 to 20/1.