A Rapid And Sensitive Radiometric Assay For Assessing The Activity Of Cytochrome P-450 (CYP) 2C9 And The Potential Of An Analyte To Inhibit CYP2C9 Activity Or Induce CYP2C9 Expression is described. All the steps of the assay, including incubations, product separation, and radioactivity counting are preferably performed in a multiwell format, which can be automated.