There are four general assay methods used to quantify a drug/biologic in a preparation, including: (1) in vivo bioassays; (2) in vitro bioassays; (3) immunoassays; and (4) receptor assays. The cell receptor assay is used to evaluate the first step in the molecular action of the drug biologic, its interaction with a specific cellular receptor. Subsequently, the drug biologic must initiate other events, such as internalisation, signal transduction, and/or alterations of one or more cellular constituents in order to elicit its biological effect. Major factors to consider in cell receptor assay development include: (1) establishment of a reference standard preparation; (2) labelling; purifying and characterisation of the biologic drug; (3) cell receptor source; (4) methodology, e.g. separation of bound and free, and other factors affecting accuracy and reproducibility; (5) ligand specificity; and (6) correlation with bioactivity. It should be emphasised that cell receptor binding cannot be assumed to correlate with biological activity because of the requirement that subsequent steps must take place prior to achieving the final response. Chemically altered drugs biologics may bind to a specific cell receptor without eliciting a biological activity. Thus, utilisation of a cell receptor assay requires careful evaluation at both the chemical and biological levels prior to its acceptance as a measure of potency.