1. Membrane-bound and free polyribosomes were isolated from parathyroid glands of normal dogs by a discontinuous density-gradient technique.
2. The conditions necessary for optimum incorporation by bound and free ribosomes of [3H]-phenylalanine into protein were determined for asays in vitro directed by both endogenous messenger ribonucleic acid (mRNA) and polyuridylic acid [poly(U)].
3. When the specific cofactors were available in optimum amounts, the rate of incorporation of amino acids into protein was directly proportional to the number of ribosomes present. This applied to assays directed by endogenous mRNA and poly-(U).
4. The results indicate that it is possible to isolate and directly study the protein synthetic activity of membrane-bound and free parathyroid ribosomes.