The standard statistical-computing package GLIM is used to analyze data from agonist-antagonist studies for the estimation of the Kd of the antagonist. The method follows the general approach of D.R. Waud in employing an iterative nonlinear least-squares curve-fitting procedure. It differs in 2 important respects: it is interactive; and it allows linear adjustments for covariates and experimental design variables. An alternative useful formulation of depature from simple competitivity and also a graphical display called a Clark plot, showing how the spacings of the dose-response lines fit the theory of simple competitive antagonism, are developed. This plot provides an alternative to the popular Schild plot for which, of necessity, the zero antagonist (control) dose-response line is heavily used both in the experimental design and in the analysis associated with it.