A bioassay (biological assay) is a method of measurement made on a living system that quantifies the effect of its exposure to a substance. Bioassays can, for example, determine precisely how much a delivered substance affects the growth of animals, plants, or microorganisms under controlled conditions, or the relative potency of a drug, enzyme, or hormone preparation on cells. See also: Biotechnology; Enzyme; Experiment; Hormone

In a bioassay, the substance being investigated can be stimulatory (such as an ion that increases the taxis behavior in protozoans) or inhibitory (such as an antibiotic that halts bacterial growth). Bioassays are typically used when the exact relationship between a substance and its presumed effect is unknown or poorly understood. These circumstances are common when numerous mechanisms or steps separate the application of a substance and the observation of a resulting change, or when the active component of a mixture needs to be pinpointed. See also: Antibiotic; Pharmaceuticals testing; Pharmacy

In creating a bioassay, researchers seek to recreate real-world conditions as much as possible during the controlled testing. Various assays may therefore involve laboratory-grown cell cultures or whole animals. In general, tests that seek to measure direct effects are more precise and better able to reflect real-world situations than indirect tests are. For example, the directly measured inhibition of fungal growth by a fungicide may be more meaningful than the extrapolation of tumor induction in humans from measurements on rats. Bioassays involving microorganisms generally demand less in time, labor, materials, and cost than ones involving larger animals do, and these considerations can determine the feasibility of potential tests. However, most scientists still deem animal bioassays to be necessary for some biomedical investigations because of the inherent complexity of animal physiology: Studying the effects of substances directly in animals is the only way to verify results in these cases. See also: Culture; Microbiology; Scientific methods