Binding studies have been performed in guinea pig and human lung membranes with the radioligand ICYP and several 'cardioselective' beta-adrenoceptor antagonists. Analysis of data from displacement curves by means of a curve fitting procedure, based on a non-linear regression analysis, indicated that in both tissues beta 1- and beta 2-type adrenoceptors coexisted. The relative proportions of the two receptor subtypes in a single tissue remained constant, irrespective of the antagonist used. The percentage of distribution between beta 1- and beta 2-adrenoceptors was about 22:78 in the guinea pig lung and 30:70 in human lung. For the compounds investigated, the affinities of beta 1- and beta 2-adrenoceptors correlated highly significantly between both species, suggesting that the 'cardioselectivity ratio" of a compound is the same in the guinea pig and in man. Mathematical treatment of the displacement curves provided no evidence of a further class of beta-adrenoceptors in lungs of both species.