Using L-[3H]dihydroalprenolol [( 3H]DHA), a potent beta-adrenergic antagonist, we demonstrated in breast cancer cells the presence of beta-adrenergic receptors with high affinity (Kd 1-9 nM) as shown by Scatchard analyses. Natural and synthetic agonists inhibited the [3H]DHA binding in the following order of potency: L-isoproterenol = L epinephrine much much greater L-norepinephrine, identical to the well-established order of potency for these compounds in producing beta-adrenergic responses. We verified that these compounds actually stimulated cAMP production in breast cancer cells. At the present time, the pathophysiological significance of beta-adrenergic receptors remains unclear. In view of the importance of cAMP in lactose production and in tumor growth mechanisms, it seems to be important to characterize the beta-adrenergic receptors in breast cancer cells in more detail and study their possible involvement in breast tumor growth.