When assessing the content of specific antibodies in serum there is tendency to abandon the specification expressed as dilution and use more reproducible data, i.e. international units. To this end it is necessary to derive from the international standard the national standard according to which positive control sera in diagnostic sets, produced by SEVAC can be described in international units. In pooled positive human serum recommended as the national standard and in the international standard first the parallel course of the dose/response curve was tested. Then both sera were repeatedly compared: into a close dilution series of the national standard one dilution of the international standard was included and in both sera the dilution with same response was found (by optic density) and thus the activity of the suggested national standard was assessed as 2400 units/ml. The author discusses also the recommended routine procedure for expressing the activity of the examined sera in international units.