biological activity, the inherent capacity of a substance, such as a drug or toxin, to alter one or more chemical or physiological functions of a cell, tissue, organ, or organism. The biological activity of a substance is determined not only by the substance’s physical and chemical nature but also by its concentration and the duration of cellular exposure to it. Biological activity may reflect a “domino effect,” in which the alteration of one function disrupts the normal activity of one or more other functions.
