Bioassays, as compared to other methods of assays (e.g. chemical or physical assay) are less accurate, less elaborate, more laborious, more troublesome and more expensive. However, bioassay is the only method of assay if (1) Active principle of drug is unknown or cannot be isolated, e.g. insulin, posterior pituitary extract etc. (2) Chemical method is either not available or if available, it is too complex and insensitive or requires higher dose e.g. insulin, acetylcholine. (3) Chemical composition is not known, e.g. long acting thyroid stimulants. (4) Chemical composition of drug differs but have the same pharmacological action and vice-versa, e.g. cardiac glycosides, catecholamines etc. Moreover, even if chemical methods are available and the results of bioassay conflict with those of the chemical assay, the bioassay is relied upon and not the chemical assay, since it is the assessment on living organism.