Specific immunosuppressive activity of cyclosporine prepared at the National Research Centre of Antibiotics by using its original producing culture was studied. It inhibited basic cellular immune responses such as DTH and transplant vs. host and prolonged the lifespan of tumor xenografs under the kidney capsule. Cyclosporine also suppressed the humoral immunity. Its inhibitory effect on lymphocyte blast transformation induced by lectins and in mixed lymphocyte cultures was shown in vitro. Cyclosporine inhibited activity of natural killer cells and T-suppressor cells induced by concanavalin A. It was demonstrated on the models of skin allotransplantation in mice and heterotopic transplantation of the heart in rats that cyclosporine prolonged the transplant lifespan. By the main indices of the cellular immunity and in the models of the transplantation immunity, cyclosporine prepared at the National Research Centre of Antibiotics was shown to be similar to that manufactured by Sandoz.