The study by BOREL et al. investigates the biological effects of cyclosporin A, a small peptide derived from the fungus *Cylindrocarpon lucidum*. Cyclosporin A is a novel antilymphocytic agent that significantly inhibits the formation of both direct and indirect plaque-forming cells in mice, demonstrating a dose-dependent effect. It also delays skin graft rejection and graft-versus-host disease in mice and rats, prevents paralysis in rats with experimental allergic encephalomyelitis, and improves symptoms in rats with established arthritis. Unlike other immunosuppressants, cyclosporin A has low myelotoxicity. The compound affects an early stage of mitogenic triggering of immunocompetent lymphoid cells rather than being cytostatic or lympholytic. The study uses various methods to assess its effects on humoral and cellular immunity, inflammatory conditions, and other pharmacological actions. Results show that cyclosporin A effectively inhibits the formation of both direct and indirect PFCs, with a prolonged effect on indirect PFCs, and is particularly effective when administered before immunization.The study by BOREL et al. investigates the biological effects of cyclosporin A, a small peptide derived from the fungus *Cylindrocarpon lucidum*. Cyclosporin A is a novel antilymphocytic agent that significantly inhibits the formation of both direct and indirect plaque-forming cells in mice, demonstrating a dose-dependent effect. It also delays skin graft rejection and graft-versus-host disease in mice and rats, prevents paralysis in rats with experimental allergic encephalomyelitis, and improves symptoms in rats with established arthritis. Unlike other immunosuppressants, cyclosporin A has low myelotoxicity. The compound affects an early stage of mitogenic triggering of immunocompetent lymphoid cells rather than being cytostatic or lympholytic. The study uses various methods to assess its effects on humoral and cellular immunity, inflammatory conditions, and other pharmacological actions. Results show that cyclosporin A effectively inhibits the formation of both direct and indirect PFCs, with a prolonged effect on indirect PFCs, and is particularly effective when administered before immunization.