Schild Plot: The Schild plot is a pharmacological method of receptor classification. To construct a Schild plot, the dose-effect curve for an agonist is determined in the presence of various concentrations of a competitive antagonist. From this experiment the pA2 is determined which is a measure of affinity of the antagonist for its receptor (i.e., the equilibrium dissociation constant). As such, the Schild Plot is sometimes referred to as pA2 analysis. Once the actual  experiments are completed a series of dose ratios (DR) are calculated for a given effect. For example the ratio of the dose of agonist (A') to produce a specific effect (e.g., half maximal effect) in the presence of the antagonist (B) to the dose required in the absence of the antagonist (A) is calculated. This is determined for several doses of antagonist and then log ((A'/A) -1) versus the negative log B is plotted (this can also be plotted versus log B, but negative log B is more convenient).  If the regression of log ((A'/A) -1) on -log B is linear with a slope of -1, then this indicates that the antagonism is competitive and by definition the agonist and antagonist act at the same recognition sites. The x-intercept of the fitted regression line is an estimate of pA2 which is the estimated equilibrium dissociation constant for the antagonist (pA2 is also the dose of antagonist that requires a 2-fold increase in agonist concentration.) The correct use of the Schild plot to estimate pA2 requires that the antagonist be a competitive antagonist. If the slope of the regression is not -1, then by definition the antagonist is not competitive or some other condition is in effect. This might include multiple binding sites or pharmacokinetic interactions. The Schild plot has a long history in Pharmacology and has proved to be useful for functional estimation of receptor properties. (see Arunklakshana & Schild, Br. J. Pharmacol. 65: 48-58, 1959).   Click here for more details on the Schild Plot