beta-adrenergic receptors (beta-ARs) were identified in CG-5 breast cancer cells using a radiometric assay. The total beta-AR concentration was measured using the highly potent beta-adrenergic antagonist (-)[3H]CGP 12177, and the densities of beta-AR subtypes were discriminated in the presence of highly selective unlabelled ligands (CGP 20712A and ICI 118551). Scatchard analysis revealed good linearity (r > 0.95) and Kd values (0.05-1 nM) indicated the presence of high affinity binding sites in CG-5 cell membranes. beta 2-AR concentrations (74%) were significantly (P < 0.05) higher than beta 1-AR concentrations (36%). Displacement studies indicated that tested adrenergic agonists displaced (-) [3H]CGP 12177 from its specific binding sites in the order of potency (-)isoproterenol > (+/-)clenbuterol > (-)adrenaline > (+/-)dobutamine > > (-)noradrenaline, whereas beta-adrenergic antagonists inhibited the binding in the following order of potency: (-)propranolol > > ICI 118 551 > > CGP 20712A. The functionality of beta-ARs identified in CG-5 cell membranes was demonstrated by the significant increase in cAMP production induced by different concentrations of isoproterenol vs unstimulated cells (control). The pathophysiological role of beta-ARs in breast cancer cells is still undefined, but their presence suggests the possible adrenergic regulation of some cellular activities such as proliferation and/or differentiation.