Crassostrea virginica, the oyster native to the gulf coast, has served as a source of ciliated epithelium for studies on the inhibitory factor in cystic fibrosis. In these studies, protein molecules with biological activity, obtained from serum, urine, saliva, and cells from cystic fibrosis homozygotes and heterozygotes, were detected, purified, and characterized. Properties of these factors are similar to those detected by other ciliated systems in mussel gills and rabbit trachea. Although none of the ciliated assays in their present stage of development offer a reliable means for heterozygote screening or for prenatal diagnosis, they represent powerful tools for characterizing biologically active molecules related to cystic fibrosis. The importance of purifying and characterizing the various cystic fibrosis factors described by several laboratories is based on evidence that their biological activities observed in vitro mimic some of the expressions of the disease observed in vivo and that the concentration of the mucociliary inhibitor in serum and fibroblast medium preparations from cystic fibrosis heterozygotes is approximately one-half of that in serum and fibroblast medium preparations from homozygotes.