Polymorphonuclear neutrophils activated by N-formylmethionyl-leucylphenylalanine are able to cross the human amnion as shown by Russo et al. (1981). With the same technique, we added polymorphonuclear neutrophils to either the epithelial or stromal surface of the amnion. The lower compartment of the incubation chamber contained 10(-8) mol/L of N-formylmethionyl-leucylphenylalanine. Of 24 amnion tested, only one showed polymorphonuclear neutrophil permeation. Ultrastructural observations indicated that the basal lamina and the zona reticularis of the basement membrane acted as barriers to polymorphonuclear neutrophil invasion. Recent studies indicate that the basal lamina and the zona reticularis of the basement membrane contain collagen type V. No data support the existence of collagenase of polymorphonuclear neutrophil origin that cleaves collagen type V. If enzymatic degradation of extracellular matrix components play a role in polymorphonuclear neutrophil invasion, it is possible that type V collagen acts as a barrier to polymorphonuclear neutrophil invasion in the human amnion.