Immunosuppressive drugs are agents capable of suppressing the development of at least one type of immune response in vivo at doses with minimal side-effects. Some characteristics regarding the mechanism of action of corticosteroids, azathioprine, cyclophosphamide, cyclosporine and monoclonal antibodies anti-CD3 are reviewed. Corticosteroids induce a redistribution of lymphocytes and display an anti-inflammatory effect; the immunosuppressive effect of azathioprine seems to consist mainly of its suppression of the inflammatory response; cyclophosphamide and cyclosporine influence the immune system itself and anti-CD3 monoclonal antibodies suppress cellular immunity fairly specifically. Finally, a brief summary of their use in renal disease, systemic vasculitis and connective tissue diseases is given.