Investigators who have previously reported attempts to utilize the isoelectric focusing (IEF) method to detect the cystic fibrosis protein (CFP) in blood samples from CF homozygotes and heterozygotes, and normal healthy controls have restricted their evaluations to serum samples. The requirement of using serum and the stringent conditions laid down for the collection, processing, and storage of serum samples have been impediments, however, to the successful use of the IEF method for CFP detection. We now report results obtained from controlled ("blind") comparative analyses of matched serum and plasma samples from CF homozygotes and heterozygotes, and normal healthy controls which indicate that the CFP is present in plasma and that heparinized plasma in particular may be a better fluid than serum for use in future studies on the CFP.