Using the oyster ciliary test, sera and fibroblast culture fluid from certain homozygotes and heterozygotes for cystic fibrosis can be shown to contain an inhibitory factor. The tissue culture fluid derived from the metachromatic fibroblast cultures (cystic fibrosis classes I and II) show ciliary inhibition. The fluid derived from the ametachromatic fibroblast cultures (cystic fibrosis class III), thus far studied, cannot be invariably distinguished from the normal noncarriers. Similarly sera from classes I and II show marked ciliary inhibition, whereas sera from class III individuals do not consistently. These experiments support the notion of genetic heterogeneity in cystic fibrosis. The nature of the inhibitory factor in tissue culture fluid and its relationship to the serum factor remain to be explored.