Detection of the cystic fibrosis (CF) heterozygote by testing for cystic fibrosis factor activity (CFFA) in serum samples using bioassays had been reported to give unreliable results. The present study was undertaken to find whether the use of medium from short term white blood cell or skin fibroblast cultures, rather than serum, as the test sample would eliminate such inconsistencies. Short term white blood cell cultures were established from 107 normal, healthy individuals. Cultures from 97 had no CFFA, six (No. 1-6) showed variable results and four (No. 7-10) had activity in all four tests done on a single sample. When the assays were repeated 3 months later on new cultures from these four positive CFFA individuals, two showed activity (Nos. 7 & 9) and two did not (Nos. 8 & 10). All 10 individuals whose cultures showed CFFA has histories of allergies. The association of CFFA in the culture medium derived from individuals having allergies with the intermittent presence of this activity suggested that environmental factors induced a CFF-like activity in such white blood cell cultures. The oyster ciliary assay, using medium from white blood cell cultures, cannot be used for screening for the CF heterozygote since the substitution of culture medium for serum did not eliminate inconsistent results and CFFA was not specific by this assay for the CF gene. Of the fibroblast lines established from the four persons whose initial white blood cell cultures consistently showed CFFA, only one (No. 7) showed the CF culture phenotype (CFFA positive and metachromasia).