A new method for estimation of agonist-affinity ( K A) and relative efficacy was introduced. This method afforded a procedure by which relative efficacy may be estimated while the actual K A values of agonist-receptor complexes are unknown. The relative efficacy may be estimated by employing a newly defined drug parameter, namely the e ES value. The e ES value is related to drug efficacy and is defined in such a manner that an isolated e ES is a meaningful quantity which may indicate whether or not spare receptors are present in an agonist–effector system. The estimation of e ES was based on the fact that fixed agonist-competitive antagonist combinations mimic partial agonists and mediate submaximal concentration–effect curves. However, for the practical estimation of e ES one may employ data acquired from agonistic concentration–effect curves determined in the absence and presence of increasing concentrations of a competitive antagonist. This procedure was illustrated by utilizing theoretical concentration–effect curves and applied practically by estimating e ES and K A values acquired from sets of carbachol and salbutamol curves. The sets of carbachol and salbutamol concentration–effect curves were determined in the absence and presence of their respective competitive antagonists, namely tripitramine and pindolol.