Steady-state concentrations of phenytoin (PHT) and carbamazepine (CBZ) were measured by a novel patient-side immunoassay system with a single-use cartridge (Biotrack 516). The Biotrack determinations were performed in whole blood and extrapolated to serum on the basis of the hemoglobin content. The results were compared with serum concentrations measured by high-performance liquid chromatography (HPLC) or the standard TDx and enzyme multiplied immunoassay (EMIT) techniques. A total of 222 samples from epileptic patients on PHT and 322 samples from patients on CBZ were analyzed. In the case of PHT there was a highly linear correlation [r = 0.985, y = 1.113x-0.589; x = HPLC, y = Biotrack] between HPLC and the Biotrack system in the concentration range of 2.5-30 micrograms/ml. In the case of CBZ, the correlation between HPLC and the Biotrack system in the concentration range of 2.0-20 micrograms/ml was somewhat lower [r = 0.931, y = 1.29x-0.136; x = HPLC, y = Biotrack]. Comparable results were also found for the correlation of the Biotrack system with the TDx assay or with the EMIT assay, respectively. Comedication had no influence, or only a minor influence (valproic acid), on the concentration of PHT and CBZ measured by the Biotrack system. Furthermore, the concentration of the metabolite carbamazepine-10, 11-epoxide had no influence on the concentration of CBZ measured by the Biotrack system. Since the automated cartridge system is simple, can be used rapidly, and is performed with only a few drops of blood, this technique offers some advantages for routine clinical use, especially under outpatient conditions.