Electrophoretic studies on purified crystalline ribonuclease showed the absence of any impurities differing in mobility from the bulk of material. The isoelectric point of ribonuclease was found by electrophoresis to be at about pH 7.8. Ultracentrifuge studies indicated fair homogeneity of ribonuclease in solution. Only one moving component has been observed. The molecular weight of ribonuclease was found to be 12,700 from rate of sedimentation (S25 = 1.85 x 10–13 in 0.5 M (NH4)2SO4) and diffusion measurement (D = 1.36 x 10–6 in 0.5 M (NH4)2SO4), in good agreement with the average value of 13,000 found from equilibrium measurements. This low value for the molecular weight of a protein would seem to discredit the value 17,600 as representing a universal unit weight for proteins in general.