We used the isoelectric focusing method developed by Wilson to analyze serum from individuals homozygous or heterozygous for cystic fibrosis. The presence of cystic fibrosis protein (CFP) was found in 37 out of 52 homozygous and 24 out of 34 heterozygous patients, which leads to a frequency of 71% for both families. Five out of 24 controls were found positive. The same study, performed on 26 plasma samples collected from the same patients, demonstrated that the detection of CFP is possible in plasma as well as in serum. Our results confirm the presence of a protein “marker” of CF in serum, but also underlines the lack of sensitivity of the isoelectric focusing technique to be used for diagnosis.