Ribonuclease crystallized in orthorhombic needles, elongated along the "c" axis; [alpha] was along a, [beta] along b, [gamma] along c; a = 36.6A, b = 40.5A, = 52.3A. There were 4 molecules per unit cell. The x-ray anhydrous molecular wt. 13.700, agreed with values obtained by other methods.