The present invention relates to a process for the enzymatic modification of a glycoprotein. The process comprises the step of contacting a glycoprotein comprising a glycan comprising a terminal GlcNAc-moiety, in the presence of glycosyltransferase that is, or is derived from, a β-(1,4)-N-acetylgalactosaminyltransferase, with a non-natural sugar-derivative nucleotide. The non-natural sugar-derivative nucleotide is according to formula (3): wherein A is selected from the group consisting of —N3, —C(O)R3, —(CH2)iC≡C—R4, —SH, —SC(O)R8, —SC(O)OR8, —SC(S)OR8, —F, —Cl, —Br —I, —OS(O)2R5, terminal C2-C24 alkenyl groups, C3-C5 cycloalkenyl groups, C4-C8 alkadienyl groups, terminal C3-C24 allenyl groups and amino groups. The invention further relates to a glycoprotein obtainable by the process according to the invention, to a bioconjugate that can be obtained by conjugating the glycoprotein with a linker-conjugate, and to β-(1,4)-N-acetylgalactosaminyltransferases that can be used in preparing the glycoprotein according to the invention.