1. Human subjects received intravenous infusions of adrenaline preceded by saline, phentolamine (an alpha-blocker) or either pronethalol or propranalol (beta-blockers), in different experiments.2. Clinical measurements of blood pressure and pulse rate confirmed that alpha- or beta-blockade had been achieved.3. The rise in clotting factor VIII induced by adrenaline was blocked by pronethalol and propranalol but not by phentolamine. This indicated that the effect of adrenaline on factor VIII was mediated by beta-receptors.