The clinical effects of a new cardioselective beta-adrenergic blocking drug, acebutolol hydrochloride (SECTRAL), were studied in 18 patients with angina pectoris, using graded treadmill testing according to a modified Bruce protocol. Measurements were made in the control state, after two weeks' treatment with placebo and after two more weeks of constant oral dose of the drug. Acebutolol produced a significant increase in the treadmill work performed before the onset of ischemic ECG changes and chest pain. The heart rate and blood pressure responses to exercise were depressed, so that the greater work load was achieved at a lower double product. Graded treadmill testing is a useful method for assessing the efficacy of treatment with a beta-blocking drug.