SummaryThe Emit Mycophenolic Acid Assay, a new homogeneous enzyme immunoassay for the quantitative analysis of mycophenolic acid (MPA) in human plasma, was evaluated and compared to a high-performance liquid chromatography (HPLC) method. Coefficients of variation (CV) of the within-run imprecision (n = 10) varied from 2.5% to 4.4% and from 1.3% to 4.9% for the Emit and the HPLC, respectively. The CV's of between-day imprecision (n = 10) ranged from 7.9% to 10.8% for the Emit and from 4.7% to 12.1% for the HPLC. Mean recoveries were 95.6% and 100.1% for Emit and HPLC, respectively. Serial dilution of a patient pool demonstrated a linear relationship between expected (x) and measured (y) concentrations: Emit,y= 0.998x+ 0.086; HPLC,y= 1.006x− 0.016. The detection limit and the lower limit of quantification were 0.087 mg/L and 0.20 mg/L for Emit. The detection limit for HPLC was 0.08 mg/L using a signal-to-noise ratio of three. Sample stability under various storage conditions was satisfactory, although storage at −20°C is recommended for storage longer than one day. No cross-reactivity from the major metabolite mycophenolic acid glucuronide (MPAG) was found. A correlation study on 261 patient samples yielded the following regression equation (bivariate Deming procedure): Emit = 1.012HPLC + 0.244,r= 0.970.