OBJECTIVE: In multiple myeloma patients, daratumumab is preferably injected subcutaneously. The summary of product characteristics of daratumumab subcutaneous injection solution specifies physicochemical stability for the prepared syringe for 24 hours at 2-8°C protected from light, and another 12 hours at room temperature (15-25°C) in ambient light conditions. The aim of this study was to determine the in-use stability of ready-to-administer daratumumab subcutaneous injection solution in different types of syringe and different conditions over a 28-day period. METHODS: Daratumumab subcutaneous (DARZALEX 1800 mg) injection solution was withdrawn into disposable three-piece Luer-Lock syringes (20 mL, 50 mL), capped, and stored light protected at 2-8°C or at room temperature (22±2°C) over a maximum period of 28 days. Samples were taken immediately after preparation (day 0) and after 2, 7, 14, 21, and 28 days. Physicochemical stability was determined by ion-exchange high-performance liquid chromatography (IE-HPLC) and size-exclusion high-performance liquid chromatography (SE-HPLC) with ultraviolet detection, pH measurement and visual inspection for particles or colour changes. RESULTS: In the IE-HPLC assay, peak areas and peak-to-peak area ratios remained unchanged over the whole study period, and showed no additional peaks of degraded daratumumab charge variants. In the SE-HPLC assay, neither a formation of aggregates nor of fragments was detected. Daratumumab monomer concentrations exceeded 95% of the initially measured concentrations over the entire test period. pH values remained constant. Test solutions remained clear, and no colour changes or visible particles were detected. All results were independent of storage conditions. CONCLUSION: Daratumumab subcutaneous injection solution proved to be physicochemically stable in capped three-piece plastic syringes for at least 28 days when stored light protected at 2-8°C or at room temperature (22±2°C). For microbiological reasons aseptic preparation and refrigerated storage are recommended. In-use stability of ready-to-administer daratumumab subcutaneous syringes prepared under appropriate aseptic conditions is given for 28 days.