A bioassay or biological assay is a biological testing procedure for estimating the concentration of a pharmaceutically active substance in a formulated product or bulk material. In contrast to common physical or chemical methods, a bioassay results in detailed information on the biological activity of a substance. Over the last decade bioassays and biostatistical analysis have become more important in effectively controlling the quality of biopharmaceutical development and manufacturing. Biological assays are typically performed in vivo or in vitro. In vivo bioassays investigate effects of a substance on a living organism, for example, injecting insulin in rabbits to measure the blood glucose level. In vitro bioassays allow a controlled environment outside the usual biological context. Typical examples are binding assays (for example ELISA), enzyme activity assays (for example spectrophotometric, fluorometric or radiometric assay), cell based assays (e.g., cell proliferation, cytotoxicity, or cell death assay) or cell and gene therapy assays.

The general approach of most bioassays is to perform a dilution assay, which measures the biological responses at several doses. 

A key assumption of a dilution assay is that the active component follows the same principle of activity in standard and in sample preparation. In this case, the unknown preparation may in theory be derived by diluting it with inert components or by concentrating the bulk solution. 

All methods are suitable for analyzing the results obtained by several biological assays. Examples of these assays are given below: