A bioassay is an analytical method used to determine the concentration or potency of a substance by its effect on living animals or plants, or on living cells or tissues. Bioassays can be either quantal or quantitative, direct or indirect. If the measured response is binary, the assay is quantal, if not, it is quantitative. Bioassays may be used to detect biological hazards or to give an assessment of the quality of a mixture. They are often used to monitor water quality as well as wastewater discharges and their impact on the surroundings. Bioassays are also used to assess the environmental impact and safety of new technologies and facilities.

Bioassays are important in the development of drugs and many biologics. In early therapy development, bioassays help determine which potential compounds have a higher likelihood of being an effective treatment. Bioassays can be cell-free (or biochemical) or cell-based procedures. High-throughput assays (also called high-throughput screenings) utilize advanced technology to miniaturize and automate the bioassay so that large libraries of potential therapies can be screened very quickly.

Bioassay procedures usually include applying a set of reagents. A reagent is a substance or compound added to a system to cause a chemical reaction or added to test whether a reaction occurs. The set of reagents used in a bioassay produces a detectable signal that can quantify the target biological activity. Signals that can be measured quantitatively include absorbance, fluorescence, luminescence, and radioactivity. Required qualities of bioassays include reproducibility, reliability, robustness, and biological relevance.

The primary value of bioassays is that they alone directly assess the biological activity of bioactive substances and products. Appropriately designed bioassays reflect the fundamental aspects of the biological activity of a bioactive molecule, including ligand-receptor binding, signal transduction processes, and the final observed biological effects. Bioassays provide valuable information concerning the potency of biological products, which is essential for evaluating batch-to-batch consistency and stability. Bioassay data are crucial at all stages in the development of biological products, from early research work to final quality control of finished products.