This document deals with the nature of WHO biological reference materials, their development for the control of the therapeutic substances, and recommendations to improve their application in diagnosis. The nature of international units specified by WHO biological reference materials is contrasted with that of SI units, and the method for assigning values in international units to such reference materials is described. The document recommends the use of SI units (mole) with existing and proposed WHO biological reference materials whenever the elementary entity of the stated component can be recognized. It also recommends that the description of quantities having no recognized kind-of-quantity with a definable dimension should be clearly distinguished by the term "arbitrary" and include a reference to the procedure and calibrator used. The WHO is urged to involve appropriate nongovernmental organizations in advising on the need for, and the suitability of, international reference materials.