Recent studies have identified abnormalities in Cl-permeation across two target cystic fibrosis (CF) epithelia (sweat duct and respiratory epithelium). In the present study, anion conductances of red blood cells (RBCs) and peripheral blood lymphocytes (PBLs) from CF and normal subjects were estimated and compared. For RBCs, the valinpmycin-induced rate constant for K+loss (PK+) was taken as an index of Pcl-For PBLs, the secondary volume increase after gramicidin pretreatment and hypotonic (0.67X isotonic) stress was used to estimate Pcl-The CI-permeabilities of RBCsand PBLsfrom CF and control subjects were comparable. These findings suggest that the abnormality in Pcl-reported for CF sweat ductal and respiratory epithelia is not expressed in circulating blood elements.