The reliability of the determination of the most common substrates with the Ektachem 700 was evaluated. Accuracy control was performed in various ways including the comparison of results with reference method values. The influence of protein concentration was investigated systematically using sera containing varying amounts of protein obtained by ultracentrifugation. Bilirubin. Deviation from the reference method values was within limits of the guidelines (Dt. Arztebl. 85 (1988) B517-B532). The influence of protein concentration was negligible. Cholesterol. The results agreed well with the reference method values. The method was not influenced by different protein concentrations. Creatinine. The results were in good agreement with reference method values, when the method was calibrated with the primary assigned values of the calibrators. At high protein concentrations, the results were higher than those of the comparative method. Glucose. The mean deviation from the reference method values was 1.0%. There was a clear positive bias at high protein concentrations. Protein. The results from the Ektachem deviated from the reference method values by -6.8%. Total glycerol (triacylglycerols). In the analysis of control sera, the Ektachem values differed greatly (+32.1%) from the reference method values. This difference was not found in a comparative study with native sera. At high protein concentrations the Ektachem results were higher than those of the comparative method. Urea. The results were lower than the method-dependent assigned values (-16.6%). This deviation was not observed in a comparative study with native sera. Interference by protein was not observed. Uric acid. In accuracy control with reference method values, a small bias was observed (+3.3%), which increased at high protein concentrations.