The present paper examines the effect of gamma radiation on the proteolytic effectiveness of pepsin of an activity of 6640, 8940 and 11,333 units in 1 g, expressed according to the Czechoslovak Pharmacopoeia. The samples were irradiated with graded doses from 5.2 to 128.7 kGy so that the course of inactivation could be determined. Effectiveness was determined six times in each sample and parallel experiments were statistically evaluated. The results are shown in Tables 1 to 3. The relative width of the interval of reliability only exceptionally exceeds 5%. A decrease in effectiveness shown in Fig. 1 shows that the course of the decrease in effectiveness did not depend in the interval used on the initial activity of the enzyme. In the semilogarithmic arrangement in Fig. 2, the dependence of the decrease in effectiveness on the dose possesses a linear course. The decrease in effectiveness in dependence on the dose of radiation can be approximately read in the graph in Fig. 3. The Czechoslovak Pharmacopoeia prescribes a dose of 24 kGy to achieve sterility by means of ionizing radiation. When microbiological indefectibility is the case, a smaller dose suffices, depending on the level of the initial contamination and the resistance of the present microorganisms. It usually does not exceed 10 kGy, in our case even 5 kGy. Nevertheless, there occurs a loss of effectivity, in the extreme case 10-13%. The largest dose used in the present paper decreased effectiveness by 60%. If ionizing radiation is to be used to decontaminate pepsin, a certain decrease in activity must be taken into account. In addition, a toxicological evaluation must be recommended.