Abstract: Excellent stabilization of polypropylene (PP) fibers exposed to sterilization levels of gamma radiation, is obtained with a hybrid stabilizer having a N-(substituted)-1-(piperazin-2-one alkyl) group at one end and a (3,5-dialkyl-4-hydroxyphenyl)-.alpha.,.alpha.-disubstituted acetamide at the other. When this hybrid of a hindered amine with a hindered phenol is combined through a disubstituted alpha carbon atom of the acetamide in a single molecule, it affords the advantages of each group with respect to its stabilization against oxidation, but without the known inherent disadvantages of either group. This molecule is N-(substituted)-1-(piperazin-2-one alkyl)-.alpha.-(3,5-dialkyl-4-hydroxyphenyl)-.alpha.,.alpha.-substituted acetamide molecule ("3,5-DHPZNA" for brevity).