WE here report the three-dimensional arrangement of the atoms in the peptide hormone, insulin, as observed in an electron density map derived by X-ray analysis at a resolution of 2· 8 A. The crystals studied were of pig insulin, which has the chemical structure shown in Fig. I, found by Ryle, Sanger, Smith and Kitsi1• They were of the rhombohedral form first obtained by Abel in 19252 and first characterized by X-ray diffraction in 19353• They have been shown-by earlier crystallographic studies-to contain two zinc atoms and six insulin molecules of weight 5,780 in the rhombohedral unit cell, tho two insulin molecules in the crystal asymmetric unit being related approximately by a non-crystallographic two-fold axis•·•. These units correspond in weight with the insulin hexamer observed in solutions containing zinc and the insulin dimer found in other solutions, particularly at acid pH. s s