Galacto-oligosaccharides (GOS) have many biological functions. They are produced from lactose by a glycosyl transfer of one or more D-galactosyl units onto the D-galactose moiety of lactose catalysed by beta-galactosidase. This reaction can proceed in batch or in continuous system. Membrane reactor with ultrafiltration ceramic membrane (150 kDa) was used and batch system as well. Lactose in phosphate buffer (197.9 g L(-1)), recombined whey (197 g L(-1)) and evaporated ultrafiltration permeate (200.7 g L(-1)) were used as substrates. The used concentration of enzyme Maxilact LX 5000 was 6 U mL(-1). The highest concentration of GOS in buffer was reached after 30 min of batch reaction and it was 25.5 g L(-1), for recombined whey it was 31.1 g L(-1) and for ultrafiltration permeate it was 32.9 g L(-1). For continuous process was obtained 8.8 g L(-1) of GUS for buffer and 21.0 g L(-1) for recombined whey and 16.7 g L(-1) of GOS for ultrafiltration permeate. (C) 2010 Elsevier Ltd. All rights reserved.