We evaluated effects of carbamazepine and its metabolite, carbamazepine 10,11-epoxide, on the measurement of tricyclic antidepressant (TCA) concentrations in serum using the fluorescence polarization immunoassay (FPIA). We determined apparent TCA concentrations in 30 patients who were receiving carbamazepine but no TCAs. Carbamazepine concentrations ranged from 1.4 to 20.9 microg/mL (5.9-88.4 micromol/L); the observed apparent TCA concentrations ranged from 31.8 to 130.1 ng/mL (113.4-463.9 micromol/L). When aliquots of the drug-free serum pool were supplemented with known concentrations of carbamazepine or its metabolite, we observed significant apparent TCA concentrations using the FPIA; however, interference of carbamazepine was more than 3-fold more than its metabolite. When serum pools prepared from patients receiving TCA but no anticonvulsant medications were supplemented with known amounts of carbamazepine or its metabolite, we observed falsely elevated TCA concentrations. We formulated an equation to calculate the apparent TCA concentration from known carbamazepine concentrations. If carbamazepine and TCAs are present in a specimen, the true TCA concentration can be estimated by subtracting the calculated TCA concentration (due to carbamazepine) from the observed TCA concentration as measured by the TCA FPIA. This mathematical modeling is feasible because TCAs, even at very high concentrations, showed no interference with the carbamazepine FPIA.