A method of producing an immunoligand/payload conjugate, which method encompasses conjugating at least one payload to an immunoligand by means of a sequence-specific transpeptidase, or a catalytic domain thereof, wherein the immunoligand comprised in the immunoligand/payload conjugate is at least one selected from the group consisting of an antibody, antibody fragment, antibody-based binding protein, or an antibody mimetic, wherein a) the antibody fragment is selected from the group consisting of (i) a Fab fragment, (ii) a F(ab')2 fragment; (iii) a heavy chain portion of a Fab(Fd) fragment, which consists of the VH and CH1 domains; (iv) a variable fragment (Fv) fragment; (v) a domain antibody (dAb) fragment, which comprises a single variable domain; (vi) an isolated complementarity determining region; (vii) a single chain FvFragment (scFv); (viii) a diabody; and (ix) a linear antibody, which comprises a pair of tandem Fvsegments (VH-CH1-VH-CH1) which, together with complementarity light chain polypeptides, form a pair of antigen binding regions; b) the antibody based binding protein is a protein that contains at least one antibody-derived VH, VL, or CH immunoglobulin domain, c) the antibody mimetic is selected from the group consisting of Ankyrin Repeat Proteins, C-Type Lectins, Transferrins, Lipocalins, 10th type III domains of fibronectin, Cysteine knots or knottins, SH-3 domains, and Fyn SH3, wherein the sequence-specific transpeptidase is a sortase enzyme which recognizes a pentapeptide recognition motif, added to a C-terminal end of the immunoligand, wherein the payload is a cytotoxic compound not exceeding a molecular weight of 2'500 Daltons that is cytotoxic to mammalian cells and impairs and kills cancer cells, and wherein the immunoligand comprises a sortase recognition motif and the cytotoxic compound is modified with a Gly n -modification, wherein n>1, and wherein the sortase recognition motif is a pentapeptide linked to the C-terminus of the immunoligand. wherein the immunoligand comprised in the immunoligand/payload conjugate is at least one selected from the group consisting of an antibody, antibody fragment, antibody-based binding protein, or an antibody mimetic, wherein a) the antibody fragment is selected from the group consisting of (i) a Fab fragment, (ii) a F(ab')2 fragment; (iii) a heavy chain portion of a Fab(Fd) fragment, which consists of the VH and CH1 domains; (iv) a variable fragment (Fv) fragment; (v) a domain antibody (dAb) fragment, which comprises a single variable domain; (vi) an isolated complementarity determining region; (vii) a single chain FvFragment (scFv); (viii) a diabody; and (ix) a linear antibody, which comprises a pair of tandem Fvsegments (VH-CH1-VH-CH1) which, together with complementarity light chain polypeptides, form a pair of antigen binding regions; b) the antibody based binding protein is a protein that contains at least one antibody-derived VH, VL, or CH immunoglobulin domain, c) the antibody mimetic is selected from the group consisting of Ankyrin Repeat Proteins, C-Type Lectins, Transferrins, Lipocalins, 10th type III domains of fibronectin, Cysteine knots or knottins, SH-3 domains, and Fyn SH3, a) the antibody fragment is selected from the group consisting of (i) a Fab fragment, (ii) a F(ab')2 fragment; (iii) a heavy chain portion of a Fab(Fd) fragment, which consists of the VH and CH1 domains; (iv) a variable fragment (Fv) fragment; (v) a domain antibody (dAb) fragment, which comprises a single variable domain; (vi) an isolated complementarity determining region; (vii) a single chain FvFragment (scFv); (viii) a diabody; and (ix) a linear antibody, which comprises a pair of tandem Fvsegments (VH-CH1-VH-CH1) which, together with complementarity light chain polypeptides, form a pair of antigen binding regions; b) the antibody based binding protein is a protein that contains at least one antibody-derived VH, VL, or CH immunoglobulin domain, c) the antibody mimetic is selected from the group consisting of Ankyrin Repeat Proteins, C-Type Lectins, Transferrins, Lipocalins, 10th type III domains of fibronectin, Cysteine knots or knottins, SH-3 domains, and Fyn SH3, wherein the sequence-specific transpeptidase is a sortase enzyme which recognizes a pentapeptide recognition motif, added to a C-terminal end of the immunoligand, wherein the payload is a cytotoxic compound not exceeding a molecular weight of 2'500 Daltons that is cytotoxic to mammalian cells and impairs and kills cancer cells, and wherein the immunoligand comprises a sortase recognition motif and the cytotoxic compound is modified with a Gly n -modification, wherein n>1, and wherein the sortase recognition motif is a pentapeptide linked to the C-terminus of the immunoligand.