The use of recommended methods has very much improved the precision and accuracy of enzymatic determinations. The use of 'enzyme reference materials' allows us to overcome the remaining difficulties. The possibility of extending standardization by using calibration materials to convert results obtained by different methods requires an essential property: 'commutability'. The relationship between patients' specimens and reference materials are conditioned by analytical causes (relative to the used methods) and clinical causes (relative to the condition of the human samples to be examined). Among the clinical causes, the different isoenzyme content of patients sera, induced by different pathologies, was considered. The effect of this influence has been investigated with an analytical approach of the isoenzymes in the individual clinical samples. The results obtained on determination of alkaline phosphatase (ALP), creatine kinase (CK), and lactate dehydrogenase (LD), are discussed according to a model of investigation which can be utilized for the experimental protocols in the assessment of commutability.