This report describes structural studies on purified polynucleotide phosphorylase from C. perfringens. A method is described for the purification of the enzyme which yields a product equivalent in activity to the native polynucleotide phosphorylase from E. coli. These studies revealed a molecular heterogeneity arising from successive stages of proteolysis, to which this enzyme is especially sensitive; unusally, the enzyme is obtained as a mixture of variable proportions of the native and proteolysed forms. We found in all cases a trimeric basic structure composed of the native (alpha) or proteolysed (lapha) or proteolysed (alpha', alpha") catalytic sub-units, However, the enzyme is rather easily dissociated into its sub-units, a phenomenon which seems to accompany proteolysis (Table). Under the action of either endogenous proteases or trypsin, two enzymatic forms are obtained: their quaternary structures seem analogous, but they differ in their catalytic properties from each other and from the initial enzyme. With some care at each step of purification, the polynucleotide phosphorylase of E. coli can be obtained exclusively in its native form. The greater susceptibility to proteolysis of the enzyme from C. perfrigens and the relationship between such degradation and quaternary structure seem to be at the origin of the peculiar behavior of this polynucleotide phosphorylase.