Two classes of cells can be subcultured from human amniotic fluid (F and AF). Ultrastructural features of these cultrued cells were determined using scanning and transmission electron microscopy with particular attention to extracellular material. Results obtained from these two cell types are compared to one another and to cultured human dermal fïbroblasts. The F-type cultures have an abundance of extracellular type i collagen fïbers, identical with cultured human dermal fïbroblasts. No type I collagen fïbers are seen in cultures of AF cells. Instead, fïne fïlamentous material is admixed with amorphous material adjacent to cell membranes where epithelial basement membrane glycoprotein has been localized. By scanning electron microscopy parallel arrays of cells are present at confluency of F cultures, typical of cultured dermal fïbroblasts. AF cultures show a much looser growth pattern at confluency. On the basis of accumulated evidence, we believe that F-type cells arise from fïbrous connective tissue and are fïbroblasts, whereas AF cells, the predominant type in early cultures used for prenatal genetic diagnosis, arise from fetal membranes and retain features of trophoblast.