Tacrolimus (FK506) is a chemotherapeutic agent, which uses calcineurin pathway via inhibiting the stimulation of T cells to prevent the formation of immune response in the recipient individual in organ transplants. FK506 is mainly metabolized in the liver by cytochrome P450 enzyme system and is known that it has high toxic effects on different cells. Mesenchymal stem cells (MSCs) have recently gained importance since their potential to be used in cellular therapy and tissue regeneration. In some clinical cases, MSCs are transferred into the patient after the organ transplantations in order to support the treatment. Because of their immunomodulatory actions and assistance to the regeneration, popularity of MSCs have been increasing recently. However, since immunosuppressive agents have a potential cytotoxic and apoptotic effect on MSCs, researches have attempted to use it as a combination with an agent that alleviates these effects. Oxytocin (OT) is primarily acting as a neuromodulator in humans and is a peptide hormone secreted by the pituitary gland of the neurohypophysis. OT has such effects on cells as to confer resistance against oxidative stress on cells and to increase the proliferation and help regeneration. Studies on the active substance of FK506 were aimed to investigate the cytotoxic effects on human adipose tissue derived MSCs.