Spin-labeled analogues of dichloroisoproterenol and propranolol were synthesized. It was found that the KD's of both probes for the beta-adrenergic receptors of frog erythrocytes were about 30-fold higher than the KD's previously reported for the parent antagonists. Thus the introduction of a bulky nitroxide moiety in place of the isopropyl group on the amino nitrogen is associated with a decrease in affinity for the beta-adrenergic receptors. Nonetheless, the affinity of the spin-labeled propranolol would appear to be within a range compatible with EPR measurements.