A highly specific homogeneous assay method for an immunosuppressive drug using an immunophilin in a single receptor format is provided. In the simplest format, a single receptor is utilized analogous to a competitive immunoassay whereby an immunophilin is substituted for an antibody, and a competition results between a drug conjugate and the drug analyte for a limited number of immunophilin binding sites. In a microparticle agglutination assay format, an immunophilin is either bound to a particle or in solution. In the case where an immunophilin is bound to a particle, a polyvalent conjugate of the drug analyte is present in solution.