Bovine pancreatic ribonuclease A (EC in aqueous solutions has been exposed to γ-ray irradiation, and several properties of the irradiated enzyme molecule were examined. Kinetic and thermodynamic parameters of RNase A were not affected even when the enzymatic activity was considerably lost by γ-ray irradiation. While the structural changes of the irradiated RNase A were detected, <tex-math>$[m^{\prime}]_{233}$</tex-math> values were gradually decreased with increasing dose. The accessibility of tyrosyl and histidyl residues in the enzyme molecule for a chemical reagent was greatly increased in relatively lower dose ranges. All six tyrosyl residues were diazotized with DHT when 70% of enzymatic activity was still retained. 2′-Cytidilate-binding capacity and enzymatic activity of RNase A were reduced depending on the increase of dose at somewhat different rates. Decrease of the heat of denaturation also indicates that the local conformation change of the enzymes through γ-ray irradiation precedes the disappearance of the enzymatic activity.

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