As previously shown ribonuclease irradiated by γ-rays under nitrogen atmosphere may be separated by column chromatography on Sephadex into two components, one having enzymatic activity, the other consisting of aggregated products with less enzymatic activity; in the presence of air an additional inactivated component is formed. Amino acid analysis of these various products shows that in all components methionine, tyrosine, phenylalanine, lysine, and histidine are destroyed with increasing dose, whereas glycine shows a small increase. In nitrogen atmosphere, besides the amino acids mentioned, cystine is changed to a great extent; these changes occur in the active component as well as in the inactivated aggregates. The following conclusions are drawn: (1) The primary structure of an RNase molecule may be changed by irradiation without loss of enzymatic activity. (2) After irradiation in nitrogen the main part of the inactivated molecules are present as aggregates, in air a small fraction is converted primarily into denatured monomers which at higher doses show aggregation, too. (3) There are no key amino acids, the alteration of which leads to the inactivation of the enzyme molecule. (4) This conclusion holds in particular for cystine; three independent proofs are presented demonstrating clearly that inactivation of ribonuclease in aqueous solution does not occur by reactive species (generated in the water) attacking the disulphide bridges and converting the molecule from the enzymatically active to the inactivated state.