Cyclosporine is a potent immunosuppressive agent that selectively impairs T cell function. The steady growth of its importance makes knowledge of its activity and effects clinically relevant. The intrinsic antibiotic activity of cyclosporine is limited; although it inhibits certain viruses, fungi, protozoa, and helminths, in practical terms these effects are insignificant. Both controlled and noncontrolled studies have shown that infection is an infrequent problem in patients with autoimmune diseases who are receiving relatively low doses. The use of cyclosporine in the setting of transplantation does not increase--and may in fact decrease--the incidence of infection in comparison with that during standard immunosuppressive therapy. Fungal, viral, and bacterial diseases remain a risk, however, and are a significant cause of morbidity in this population. Antimicrobial therapy may alter cyclosporine levels and cause nephrotoxicity or graft rejection. Careful monitoring of cyclosporine levels during antimicrobial treatment is therefore recommended.