The present invention relates to non-isotopic immunoassays for thiopurine methyltransferase (TPMT). The immunoassays of this invention may be homogenous or heterogenous, in which detection of the TPMT-catalyzed reaction product relies upon specific binding of antibody to 6-MMP or other TPMT-catalyzed reaction products. Preferred embodiments of this invention include a Rapid Immunomigration Cassette and an assay carried out in ELISA assay format.