Mesenchymal stem/stromal cells (MSCs) offer promising therapeutic potential in cell-based therapies for various diseases. However, the safety of genetically modified MSCs remains poorly understood. This study aimed to evaluate the general toxicity and safety of Wharton's Jelly-Derived MSCs (WJ-MSCs) engineered to express the antimicrobial peptide SE-33 in an animal model. Genetically modified WJ-MSCs expressing SE-33 were administered to C57BL/6 mice at both therapeutic and excessive doses, either once or repeatedly. Animal monitoring included mortality, clinical signs, and behavioral observations. The toxicity assessment involved histopathological, hematological, and biochemical analyses of major organs and tissues, while immunotoxicity and immunogenicity were examined through humoral and cellular immune responses, macrophage phagocytic activity, and lymphocyte blast transformation. Antimicrobial efficacy was evaluated in a