The effect of β-receptor blocking agents on [14C] amino acid incorporation into protein in cultures of chick embryo liver cells was studied. DL-Propranolol, a β-blocker with non-specific membrane effects, caused a 40 per cent inhibition of incorporation of [14C] amino acids into protein. The inhibition was concentration dependent and reversible. A similar inhibition was obtained by oxprenolol (Trasicor), which is also a β-receptor blocking agent with non-specific membrane effects, and by the membrane active compounds: D-propranolol, lidocaine and quinidine. Pindolol (Visken), and practolol, which are almost devoid of membrane activity, were ineffective. These data indicate that the inhibitory effect of dl-propranolol and oxprenolol on protein synthesis is caused by their non-specific membrane effects.