This paper deals with the study of poly(ethylene terephthalate) (PET) fiber modification by antimicrobial substances following the crazing mechanism. The highest mean number of crazes and their maximal degree of opening in PET fibers subjected to tension in surface-active bactericidal liquids are reached when the plastic flow of PET begins and necks appear on the fibers. Further straining of the fibers results in a gradual transformation of the material into a fibrillar structure of strands that close the crazes. It is established that the amount of the bactericidal liquid captured by the crazes is 0.9 to 3.3 wt.% of the PET material and that the liquid localizes predominantly in the polymer amorphous phase. PET fibers treated by the developed liquids using the procedure developed preserve their antimicrobial properties after 10 to 50 cycles of heat-wet treatment.