A natural specific intracellular ligand as a reagent for cyclosporine blood level determination.

We isolated and characterized a non-immunoglobulin fraction from calf thymuses which was found very active in binding [3H]labelled cyclosporine (CsA). It contains the natural, intracellular receptor cyclophilin, specifically binding to the active site of the CsA molecule, which may also play a part in the immunosuppression of human immunocompetent cells. This material with a high selectivity could be used, instead of specific monoclonal or polyclonal antibodies, to determine CsA levels in patient's blood.