Cystic fibrosis (CF) is one of the most common recessively inherited genetic diseases in Caucasian populations. It was first formally described as a distinct disorder in 1938 (Andersen, 1938), and has subsequently been the subject of intensive clinical and laboratory investigations. Despite the barrage of experimental work on the disorder, the primary protein lesion in CF remains unknown. The medical literature is awash with reports on significant biochemical and physiological abnormalities in CF homozygotes, but few of these have turned out to be consistently reproducible and none has yet led to a consensus on an area of metabolism that might be the source of an aberrant protein or enzyme. A possible exception to this statement is recent work by Quinton and colleagues (Quinton, 1983; Bijman and Quinton, 1984) identifying a chloride ion transport abnormality in epithelial cells dissected from the ducts of isolated sweat glands. Though these observations are physiological rather than biochemical, they have begun to make sense of the pathology of CF and may soon lead to identification of a primary protein abnormality.