Ribonuclease A was irradiated in water with ⁶⁰Co gamma radiation, and the products formed were separated according to their molecular weight by column chromatography on Sephadex G-50. When the irradiations are carried out in nitrogen atmosphere aggregation to dimers and higher polymers is observed. An appreciable fraction of this aggregated component retains enzymatic activity. At higher doses the enzymatic activity of the aggregates is inactivated at the same rate as monomer ribonuclease A. Irradiation in air yields two components; one is equivalent to that found in nitrogen atmosphere, the other has an apparent molecular weight of about 20 000 but contains no enzymatic activity. This last component is not observed when methionine is present during irradiation. In methionine containing solutions all inactivited ribonuclease molecules exist in the form of dimers and polymers. Irradiation in the dry state leads to the same result. Consequently, every model designed to describe the radiation action on ribonuclease has to consider the fact that in solution, in the presence of a protecting agent, and in the dry state the loss of enzymatic activity is always accompanied by aggregation to products with increased molecular weight.