The constrained four parameter logistic model has found wide application in describing dose response relationships across many assay systems. This discussion examines the basic model and its practical application to potency testing in the context of the 96 well plate. A two step procedure is recommended for the analysis: (i) the constrained logistic model to generate potency estimates, (ii) a linear mixed-effects model to account for within-plate and between plate variability for producing the final combined estimate of potency. The method is outlined in a case study. Design issues related to possible location effects on the plate may be ameliorated by use of a Latin square design.