Scanning electron microscopy was used to demonstrate the process of moist bacterial strike-through of woven and nonwoven surgical materials. Three woven and three nonwoven materials were challenged with an aqueous suspension of Serratia marcescens. The results of these studies confirmed that relatively new, less than 100 cycles of washing and sterilizing, 270 thread Quarpel treated Pima cotton prevents moist bacterial penetration. However, this same woven material when washed and sterilized more than 100 times allowed bacterial penetration. Nonwoven materials prevented penetration only when they were impregnated with plastic or reinforced with a plastic film. Prevention of moist bacterial strike-through of surgical materials, whether they be woven or nonwoven, is dependent upon the effectiveness of their waterproof quality. In woven materials, we have confirmed our previous findings indicating that loss of waterproof characteristics, which occurs after 75 washing-sterilizing cyclings, leads to permeability and to moist bacterial strike-through, regardless of the weave density. In nonwoven materials, dependable resistance to moist bacterial strike, through was achieved only when all moisture penetration was prevented by reinforcement with waterproof plastic film.