The binding of [³H](−)-alprenolol (a potent β-adrenergic antagonist) to sites in frog erythrocyte membranes has been studied by a centrifugal assay. The specificity of the binding sites is strikingly similar to what might be expected of the β-adrenergic receptor binding sites which mediate stimulation of adenylate cyclase by catecholamines in these membranes. The sites bind β-adrenergic antagonists and agonists with affinities which are directly related to their antagonist or agonist potency on the frog erythrocyte membrane adenylate cyclase. Binding shows strict stereospecificity with (−)-isomers exhibiting two orders of magnitude higher affinities than (+)-isomers. Dissociation constants for potent β-adrenergic antagonists are in the range of 10⁻⁹ – 10⁻⁸M whereas those for β-adrenergic agonists are about two orders of magnitude higher (≥ 10⁻⁶M).