In this paper there are investigated conditions of the lactose hydrolysis and effective synthesis of galactooligosaccharides in permeate after the completed process of milk ultrafiltration with a β- galactosidase preparation added. A galactooligosaccharide concentrate obtained was then used to produce yogurt. It was assessed what impact on the hydrolysis degree of lactose, as well as on the effectiveness of synthesis of galactooligosaccharides had the temperature § time of the reaction, further, the presence/absence of proteins in permeate solutions, and concentration levels of lactose in permeate at the beginning of the process. It was stated that if the process was conducted in an under-proteinized permeate with proteins containing 30% of lactose at 5°C during a period of 8 hours, the effectiveness of galactooligosaccharides synthesis was satisfactory. The concentrate of galactooligosaccharides was used to enrich dry matter contained in milk destined for the further production of yogurts. It was stated that the addition of galactooligosaccharides stimulated the growth of bacteria belonging to the Lactobacillus bulgaricus and Streptococcus thermophilus species.