An enzyme-linked immunosorbent assay (ELISA) has been developed which allows hybridoma cell cultures to be tested for the presence of monoclonal antibodies specific for cyclosporin A. Immunization of mice with free cyclosporin was found to be preferable to immunization with cyclosporin-carrier conjugates. It is hoped that the availability of monoclonal antibodies to cyclosporin will clarify the contribution of cyclosporin metabolites to immunosuppression and nephrotoxicity.