Two hundred and eighty-two patients less than 66 years of age admitted with suspected or definite myocardial infarction were allocated in a random fashion to treatment with alprenolol or placebo. Treatment was started immediately upon admission with 5-10 mg alprenolol or placebo intravenously, followed by 200 mg alprenolol or placebo orally twice a day for one year. In 178 patients a definite myocardial infarction was diagnosed. Myocardial infarct size could be estimated from serial determinations of serum total creatine kinase in 42 patients treated with alprenolol and 43 patients receiving placebos. Median infarct size was 20.6 CK g Eq/m2 body surface in the alprenolol group, the corresponding figure in the placebo group being 34.4 CK g Eq/m2 body surface. Median rate of release of creatine kinase from the ischaemic myocardium was 27.7 U/1 per hour and 48.0 U/1 per hour after alprenolol and placebo, respectively. Alprenolol limited infarct size significantly provided the treatment was started within 12 hours of the onset of symptoms.