One hundred and four serum samples submitted to our laboratory for total phenytoin levels were also assayed for free phenytoin levels using the Abbott TDx and AxSYM immunochemistry analyzers (Abbott Laboratories; Abbott Park, IL, U.S.A.). Free phenytoin levels were assessed using ultrafiltrates obtained with the Centrifree Microfiltration system (Amicon, Inc.; Beverly, MA, U.S.A.). There was an acceptable correlation of free phenytoin levels between the two instruments (r= 0.994; mean value of free fraction was TDx = 1.7 µg/mL and AxSYM = 1.6 µg/mL). The mean free phenytoin values correlated well with previously reported values in the literature. Acceptable recovery (88.8%-101.5%) and precision (CV = 2.3%-11.0%) were also obtained. The results of this study indicated that free phenytoin levels can be obtained with acceptable accuracy and precision using the total phenytoin reagents.