1. n. 
A laboratory test or other assessment utilizing a living organism, such as mysid shrimp, to determine the effect of a condition to which the organism is exposed. Such tests are performed under controlled environmental conditions and duration. Bioassay tests of drilling fluids are required by governmental agencies throughout the world prior to discharge of mud or cuttings. The organisms used in bioassays are those found in the area that would be most affected by contact with the proposed drilling fluid. The dosage of interest is typically the lethal concentration, known as LC50, that will kill 50% of the population of organisms in a given period of time. Chronic bioassay tests indicate sublethal effects, such as changes in growth or reproduction of the organism over a longer period of time.