Serum samples from cystic fibrosis homozygotes and heterozygotes contain elevated levels of a protein, known as cystic fibrosis antigen, which is synthesized primarily in granulocytes. We have produced monoclonal antibodies against cystic fibrosis antigen and developed a sensitive two-site immunoassay. Antigen levels were evaluated in serum samples from 50 cystic fibrosis homozygotes, 34 heterozygotes, 60 healthy controls and 25 disease controls. Simultaneous measurement was made of another granulocyte-derived serum protein, lactoferrin, and also of the acute-phase reactant, C-Reactive Protein. Attempts to use the concentrations of cystic fibrosis antigen in serum to distinguish cystic fibrosis patients from heterozygotes were unsuccessful, even when these concentrations were expressed as a ratio with lactoferrin or with C-Reactive Protein. However, examination of the ratio of cystic fibrosis antigen to lactoferrin in serum samples from cystic fibrosis heterozygotes suggests that there is some specific association between this antigen and the cystic fibrosis gene.