An immunoconjugate for use in treating a disease associated with target cells expressing CD138 in a human subject, wherein the immunoconjugate comprises at least one engineered targeting antibody targeting CD138 expressing cells, and at least one effector molecule, wherein said engineered targeting antibody is functionally attached to said effector molecule to form said immunoconjugate, wherein preferably at least a part of the engineered targeting antibody confers IgG4 isotype properties, wherein the effector molecule is at least one maytansinoid, wherein the engineered targeting antibody comprises: (i) a heavy chain variable region CDR1 comprising amino acid residues 31 to 35 of SEQ ID NO: 1, a heavy chain variable region CDR2 comprising amino acid residues 51 to 68 of SEQ ID NO: 1, and a heavy chain variable region CDR3 comprising amino acid residues 99 to 111 of SEQ ID NO: 1; and (ii) a light chain variable region CDR1 comprising amino acid residues 24 to 34 of SEQ ID NO: 2, a light chain variable region CDR2 comprising amino acid residues 50 to 56 of SEQ ID NO: 2, and a light chain variable region CDR3 comprising amino acid residues 89 to 97 of SEQ ID NO: 2; wherein the immunoconjugate is to be administered in a multiple dose regimen, wherein the aggregate dose to be administered within an active treatment cycle is an aggregate maximum tolerable dose (AMTD) or a fraction of the AMTD and wherein said AMTD and/or said fraction exceeds the dose resulting in dose limiting toxicity when the immunoconjugate is administered as a single dose, including as part of a multiple single dose regimen and/or exceeds the maximum tolerable dose when the immunoconjugate is administered as a single dose, including as part of a multiple single dose regimen within said active treatment cycle, wherein the aggregate maximum tolerable dose is at least 240 mg/m 2, preferably 300 mg/m 2, preferably 360 mg/m 2, preferably 420 mg/m 2, wherein the dose resulting in said dose limiting toxicity is 200 mg/m 2 and/or the maximum tolerable dose is at least 160 mg/m 2 or at least 180 mg/m 2, wherein the immunoconjugate is to be administered in an active treatment cycle of 21 days, at least three times within said 21 days, wherein the active treatment cycle includes administration of the immunoconjugate being performed at least once a week, wherein the disease associated with target cells expressing CD138 is multiple myeloma, renal cell carcinoma, endometrial cancer, cervical cancer, prostate adenocarcinoma, pancreatic carcinoma, gastric cancer, bladder cancer, mammary carcinoma, hepato-carcinoma, colorectal carcinoma, colon carcinoma, squamous cell carcinoma, lung cancer, non Hodgkin lymphoma, thymus carcinoma, uterus carcinoma, urinary carcinoma or ovarian carcinoma. (i) a heavy chain variable region CDR1 comprising amino acid residues 31 to 35 of SEQ ID NO: 1, a heavy chain variable region CDR2 comprising amino acid residues 51 to 68 of SEQ ID NO: 1, and a heavy chain variable region CDR3 comprising amino acid residues 99 to 111 of SEQ ID NO: 1; and (ii) a light chain variable region CDR1 comprising amino acid residues 24 to 34 of SEQ ID NO: 2, a light chain variable region CDR2 comprising amino acid residues 50 to 56 of SEQ ID NO: 2, and a light chain variable region CDR3 comprising amino acid residues 89 to 97 of SEQ ID NO: 2;