β-adrenergic receptors (β-ARs) were identified in CG-5 breast cancer cells using a radiometric assay. The total β-AR concentration was measured using the highly potent β-adrenergic antagonist (-)[ 3 H]CGP 12177, and the densities of β-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) andK d values (0.05-1 nM) indicated the presence of high affinity binding sites in CG-5 cell membranes. β 2 -AR concentrations (74%) were significantly (P<0.05) higher than β 1 -AR concentrations (36%). Displacement studies indicated that tested adrenergic agonists displaced (-)[ 3 H]CGP 12177 from its specific binding sites in the order of potency (-)isoproterenol>(+/-)clenbuterol>(-)adrenaline>(+/-)dobutamine >(-)noradre naline, whereas β-adrenergic antagonists inhibited the binding in the following order of potency: (-)propranolol ICI 118 551 CGP 20712A. The functionality of β-ARs identified in CG-5 cell membranes was demonstrated by the significant increase in cAMP production induced by different concentrations of isoproterenolvsunstimulated cells (control). The pathophysiological role of β-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.