In order to provide quality assurance in the field of clinical chemistry, required by European Standards, the Romanian national metrological infrastructure has to be improved. The main objectives of this important activity are metrological assurance of concentration measurements, the assessment of the uncertainty, and the traceability of measurements. Within this framework, reference materials play an important role. The experience of INM in the preparation, certification and use of reference materials and in metrological assurance of clinical laboratory measurements (especially when absorption photometric devices are used) is described. The combined uncertainty associated with chemical preparation of these reference materials is estimated.