Trans-galactosylation of protected serine and threonine by beta-galactosidase is the first rational step in the in vitro glycosylation of peptides and proteins. The experiments show the potential of trans-galactosylations with lactose for applications in which no product purification (such as that used in the food and life science industries) is needed. To examine these glycosylations systematically, the protected amino acids Boc-SerOMe, Boc-Thr-OMe, Cbz-Ser-OMe, and Cbz-Thr-OMe are used as substrates. The trans-mono-galactosylation of serine with an excess of lactose yields 28% of N-tert-butoxycarbonyl-1-O-beta-D-galactopyranosyl-L-serine-methylester. The same transformational conditions, when applied to threonine, produced N-tert-butoxycarbonyl-1-O-beta-D-galactopyranosyl-L-threonine-methyleste r in lower quantities. Mono-galactosylated serine and threonine are further galactosylated in the examined experimental setup to yield bi-galactosylated products also, especially at 50 degrees C with completely dissolved lactose.