Assays employed in the biological sciences fall into two categories, which may be respectively termed "comparative" (or "functionally-specific") and "analytical" (or "structurally-specific"). The former are intended to compare the relative effects of substances, or mixtures of substances--not necessarily of identical chemical structure--on a biological system (e.g. whole animal, tissue, cell, etc). Results are represented by units of effect (i.e. they are not units of "amount" of the substance(s) measured), and differ depending on the biological system used. Such assays cannot be "standardised" by the use of a calibrant. In contrast, analytical assays are intended to measure the number of molecules (or mass) of a single substance of unique chemical structure in a test sample, and cannot legitimately be employed to measure mixtures of substances of different structure. Results are represented by units of molecular number or mass, and should be identical for any test sample irrespective of the assay system used. Immunoassays generally fall into this category. Insofar as the antigenic substances present in standards or test samples are dissimilar and/or molecularly heterogeneous, an immunoassay is invalid, and the results it yields have no universal significance. Attempts to standardize "analytically-invalid" immunoassays inevitably fail. Many substances of biological interest (e.g. TSH)--initially defined in terms of their biological function--have subsequently been shown to be molecularly heterogenous. Problems thus arise in the standardization of immunoassays used for their measurement, reflecting the fact that the measurement of a mixture of substances of differing molecular structure (and function) is a meaningless concept. It is thus impossible to "measure TSH"; it is only possible to measure the effect TSH exerts in a particularly assay system. The only long-term solution to this problem is the development of assay systems measuring individual components of such heterogenous mixtures.