The term dilution assay is generally used to designate a special type of bioassay in which one or more preparations (e.g. a drug) are administered to experimental units   at different dose levels inducing a measurable biological response. The dose levels are prepared by dilution in a diluent that is inert in respect of the response. The   experimental units can for example be cell-cultures, tissues, organs or living animals. The biological response may be quantal (e.g. positive/negative) or quantitative (e.g.   growth). The goal is to relate the response to the dose, usually by interpolation techniques, and in many cases to express the potency/activity of the test preparation(s)   relative to a standard of known potency/activity.
Dilution assays can be direct or indirect. In a direct dilution assay the amount of dose needed to produce a specific (fixed) response is measured, so that the dose is a   stochastic variable defining the tolerance distribution. Conversely, in an indirect dilution assay the dose levels are administered at fixed dose levels, so that the   response is a stochastic variable.
In some assays, there may be strong reasons for believing that all the constituents of the test preparation except one, are without any effect on the studied response of the subjects. An assay of the preparation against a standard preparation of the effective constituent, is then equivalent to an analysis for determining the content of the constituent. This may be described as analytical dilution assay.