The binding of (-)-[125I]pindolol ((-)-[125I]PIN) to human platelet membranes has been examined and identifies a relatively low density of beta-adrenoceptor binding sites in this tissue. The relative order of potency of the catecholamines in displacement experiments, isoprenaline greater than adrenaline greater than noradrenaline and curve-fitting analysis of the competition binding isotherms using subtype selective beta-adrenergic antagonists, suggests the presence of an homogeneous population of beta 2-adrenoceptors. The effects of guanine nucleotides and mono- and divalent cations on the isoprenaline displacement of (-)-[125I]PIN binding to human platelet and rat lung membrane beta-adrenoceptors was compared in parallel experiments. In the presence of Mg2+ the agonist showed relatively lower overall affinity for the platelet receptor with a steep displacement curve, Hill slope approaching unity. Under these circumstances Gpp(NH)p characteristically produced a shift to the right and a steepening of the agonist displacement curve in rat lung but had no effect in platelet membranes. NaCl caused a parallel shift to the right of the curves in both systems in the presence or absence of Gpp(NH)p. The significance of these results in relationship to the efficiency of coupling of the human platelet beta-adrenoceptor to adenylate cyclase is discussed.