1. In chemistry, the determination of the concentration of a chemical by its effect on the growth of an organism under experimental conditions.
2. In toxicology, the use of a live organism or cell culture to test for the presence of a chemical.
3. In virology, testing a virus by finding out how infective it is for a given host.

The information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary. For copyright details, see the BioTech homenode.