Process for the preparation of an antibody-conjugate, comprising the steps of: providing an IgG antibody comprising at least two N-linked glycosylation sites on the combination of a single heavy chain and single light chain; and trimming an oligosaccharide that is attached to a glycosylation site, by the action of an endoglycosidase, in order to obtain a proximal N-linked GlcNAc-residue at said glycosylation site; and optionally repeating step in order to trim an oligosaccharide that is attached to a different glycosylation site; and attaching a monosaccharide derivative Su x to said proximal N-linked GlcNAc-residue, in the presence of a galactosyltransferase or a galactosyltransferase comprising a mutant catalytic domain, wherein Su x is defined as a monosaccharide derivative comprising x functional groups A wherein x is 1, 2, 3 or 4 and wherein A is selected from the group consisting of an azido group, a keto group, an alkynyl group, a thiol group or a precursor thereof, a halogen, a sulfonyloxy group, a halogenated acetamido group, a mercaptoacetamido group and a sulfonylated hydroxyacetamido group, in order to obtain a proximal N-linked GlcNAc-Su x substituent at said N-glycosylation site; and optionally: repeating step, in order to trim an oligosaccharide that is attached to a different glycosylation site; and repeating step; and reacting said proximal N-linked GlcNAc-Su x substituent with a linker-conjugate, wherein said linker-conjugate comprises a functional group B and a molecule of interest D, wherein said functional group B is a functional group that is capable of reacting with a functional group A of said GlcNAc-Su x substituent, and wherein Su x is defined as above, with the proviso that A is not a thiol group precursor; and optionally: repeating step in order to trim an oligosaccharide that is attached to a different glycosylation site; and repeating step; and repeating step; and wherein the proximal N-linked GlcNAc-residue in steps, and is optionally fucosylated; wherein the process comprises one of steps, and. providing an IgG antibody comprising at least two N-linked glycosylation sites on the combination of a single heavy chain and single light chain; and trimming an oligosaccharide that is attached to a glycosylation site, by the action of an endoglycosidase, in order to obtain a proximal N-linked GlcNAc-residue at said glycosylation site; and optionally repeating step in order to trim an oligosaccharide that is attached to a different glycosylation site; and attaching a monosaccharide derivative Su x to said proximal N-linked GlcNAc-residue, in the presence of a galactosyltransferase or a galactosyltransferase comprising a mutant catalytic domain, wherein Su x is defined as a monosaccharide derivative comprising x functional groups A wherein x is 1, 2, 3 or 4 and wherein A is selected from the group consisting of an azido group, a keto group, an alkynyl group, a thiol group or a precursor thereof, a halogen, a sulfonyloxy group, a halogenated acetamido group, a mercaptoacetamido group and a sulfonylated hydroxyacetamido group, in order to obtain a proximal N-linked GlcNAc-Su x substituent at said N-glycosylation site; and optionally: repeating step, in order to trim an oligosaccharide that is attached to a different glycosylation site; and repeating step; and repeating step, in order to trim an oligosaccharide that is attached to a different glycosylation site; and repeating step; and reacting said proximal N-linked GlcNAc-Su x substituent with a linker-conjugate, wherein said linker-conjugate comprises a functional group B and a molecule of interest D, wherein said functional group B is a functional group that is capable of reacting with a functional group A of said GlcNAc-Su x substituent, and wherein Su x is defined as above, with the proviso that A is not a thiol group precursor; and optionally: repeating step in order to trim an oligosaccharide that is attached to a different glycosylation site; and repeating step; and repeating step; and repeating step in order to trim an oligosaccharide that is attached to a different glycosylation site; and repeating step; and repeating step; and