In this procedure, tentative calibration values are assigned to secondary standards by analysis of that material with the routinely used instrument calibrated by use of the former lot of material. Aliquots of serum from patients are then analyzed by the routine method and an appropriate calibration method in which primary standards are used. If the patient-sera results by both methods are the same, the assigned calibration value is assumed to be correct; if not, the calibration value of the secondary-standard material is adjusted to produce agreement in patient-sera results between the two methods. Thus, calibration of the routine method is linked via the calibration method to primary standards as the criterion of accuracy. This approach corrects for matrix interference of serum-based secondary standards. All that is expected for the secondary standard to obtain an accurate result for patient sera is to produce an analytical signal (irrespective of the origin of the signal) that is equivalent to the calibration value. Differences were significant between the secondary-standard calibration value we determined by this procedure and that assigned by the manufacturer for bilirubin, calcium, phosphorus, chloride, CO2, creatinine, glucose, sodium, uric acid, and commonly measured enzymes.