Serum from cystic fibrosis patients and their parents will inhibit ciliary movement on oyster gill tissue. Previous studies have indicated the factor responsible for ciliary inhibition is a cationic protein with a molecular weight between 125,000 to 200,000, and has properties similar to γG‐globulin. To support this evidence, sera from CF patients, heterozygotes, and control individuals were subjected to rivanol precipitation and the supernatants tested on oyster cilia. In addition other sources of cilia and flagella were examined as possible assay systems for the CF fact