Tubulointerstitial nephritis, regularly induced in Brown Norway rats by autosensitization with tubular basement membranes, appeared acutely at about 10 days and was completely inhibited by Cyclosporin A (CyA) when this was given before and at the time of onset of the disease. It did not recur after the end of therapy. In spite of this disease inhibition, autoantibodies against tubular basement membranes persisted in the circulation and in the kidney. It is therefore suggested that it is not so much the antibodies that are the decisive factors in the pathogenesis of this and other autoimmune diseases but, rather cellular factors that are directly influenced by CyA. Although in this model CyA application was a preventive measure, the experiments provide a cogent reason for the application of CyA as early as possible in acute and seriously damaging diseases.