Saturation experiments using increasing concentrations of radioligand are commonly used to determine receptor number and affinity, but this protocol is not feasible in all situations. Alternatively, competitive binding experiments are often performed in which binding of a single concentration of radioligand is completed for by multiple concentrations of the same unlabelled ligand, but the analysis of such data has been difficult. Antonio DeBlasi and colleagues present here a simple method for calculating receptor number and affinity from competitive binding data. This experimental protocol is useful when a single class of binding site is present, but is often not able to detect the presence of two classes of site.