The analysis of mycophenolic acid (MPA) has proved a valuable adjunct to the clinical care of organ transplant recipients. The analytic validation of the enzyme multiplied immunoassay technique (EMIT) for the determination of MPA in plasma is described. The EMIT MPA standard curve was 0 to 15.0 microg/mL, and curve storage was maintained for 4 weeks. The MPA EMIT assay proved reliable and reproducible, as shown by the intra-assay and interassay coefficients of variation (1.58-3.68% and 1.23-7.57%, respectively). Excellent linear correlation ( r = 0.999) was observed for dilution linearity. The sensitivity of the assay was 0.01 microg/mL. Recoveries of 99.4% to 104.2% were obtained by spiking aliquots of three controls of known MPA concentrations with MPA. No interference was observed for endogenous substances and coadministered immunosuppressant drugs, and no cross-reactivity from the major metabolite MPA glucuronide was found. The high-performance liquid chromatography (HPLC) assay used protein precipitation and C18 ion-pair chromatography with ultraviolet detection at 304 nm. Plasma concentrations of MPA were measured using EMIT and HPLC. A linear relationship was observed between EMIT and HPLC (EMIT = 1.091 x HPLC - 0.089; r 2 = 0.990, n = 129). These results indicate that EMIT is a simple, rapid, and sensitive assay method for the measurement of MPA in plasma.