Bioassays using ciliary systems have detected a factor or factors in cystic fibrosis (CF) sera and tissue culture medium derived from CF cells. The typical shortcomings of an assay measuring biological activity have been studied, and the means to overcome the weaknesses of the oyster gill cilia assay have been established. The presence of the cystic fibrosis mucociliary inhibitor (CFMI) in experimental fractions may be determined by accepting data from only those assays in which authentic CF and normal (non-CF) fractions give defined reactions, by measuring the reaction of each sample at least three times, and by examining each experimental sample at a protein concentration greater than the minimum established in this study. The relative concentrations of the CFMI present in the first steps of purification of serum and medium have been calculated in terms of units of inhibition. Generally, the units of inhibition present in serum and medium fractions from heterozygotes are close to one-half of that in fractions from homozygous sources. Analogous fractions concentrated from a normal (non-CF) source never inhibited mucociliary activity, even when tested at nearly 100 times the CF concentration. Ciliary assays utilizing oyster gills are essential for monitoring fractionation procedures aimed at purifying the CFMI, and have been shown to be capable and reliable enough to do so.