The molecular structure of tRNA is being studied by x-ray diffraction techniques. Heavy atom derivatives have been found and an electron density map calculated to 3.0 Angstrom units resolution. The map has been interpreted in terms of tRNA tertiary structures. Crystals have been obtained which yield 2.3 Angstrom units resolution. The mechanism of the ribosomal peptidyl transferase is being studied in E. coli and in chick ribosomes. Muscle protein biosynthesis is also being studied in chick embryos and in tissue culture using mouse cells.