Temperature-converting factors for six enzymes, which are determined very often in the clinical laboratory, are given. The determinations were carried out according to the recommendations of the German Society for Clinical Chemistry and of the Committee on Enzymes of the Dutch Society for Clinical Chemistry. These factors are described not only for human sera but also for some batches of commercial test sera and for the pure enzymes. There are two main conclusions. (1) Factors determined according to the German recommendations cannot be used in any simple way to convert enzymatic activities which have been determined according to the Dutch recommendations. (2) Factors determined using commercial test sera cannot be used for converting enzymatic activities of human origin.