For a variety of reasons, pharmacologists often construct 'inhibition curves' to evaluate antagonists rather than employing the more comprehensive 'Schild analysis'. There is a widespread perception that it is only possible to derive antagonist Kb values from such experiments via application of the Cheng-Prusoff equation, requiring knowledge of the agonist affinity at the receptor. In this article, Douglas Craig presents a practical examination of 'inhibition curve' methodology and demonstrates that it is a related equation requiring no knowledge of agonist affinity, rather than the Cheng-Prusoff equation, that affords theoretically valid estimates of antagonist Kb values from this technique.