The characteristics of beta-adrenoceptors on frog and chick erythrocytes have been compared to those present on rat erythrocyte, lung and cerebral cortical membranes. The receptors have been examined using the specific ligand [3H]dihydroalprenolol ([3H]DHA) with conventional filtration techniques. The equilibrium dissociation constant (KD) of [3H]DHA and the affinities of the non-selective beta-adrenoceptor antagonists propranolol, timolol and alprenolol, were very similar in all preparations examined. The relative order of potency of the catecholamines, isoprenaline greater than adrenaline greater than noradrenaline in rat lung, rat erythrocyte and frog erythrocyte; isoprenaline greater than noradrenaline greater than or equal to adrenaline in chick erythrocyte and rat cortex, suggest an overall beta 2-classification for the former and beta 1-classification of the latter tissues. However, the use of highly selective beta 1- or beta 2-agents and computer-assisted iterative curve fitting revealed that whereas rat cortex and lung possess both receptor subtypes (lung 80% beta 2, 20% beta 1: cortex 65% beta 1, 35% beta 2) only beta 2-sites were observed in rat erythrocytes. On the other hand, chick and frog erythrocytes possess a homogeneous population of receptors that do not strictly correspond either to beta 1- or beta 2-adrenoceptors. The atypical nature of these non-mammalian erythrocyte beta-adrenoceptors is discussed in relation to their previous extensive use as model beta-adrenoceptor systems.