We have developed a standardized biophysical assay for the rapid detection of individuals homozygous or heterozygous for cystic fibrosis (C/F). The assay employs isoelectric focusing in thin layer polyacrylamide gels to analyze microliter quantities of whole serum for the presence of a C/F factor protein and for deletions in a group of proteins called proteins B, C, and D (Fig, 1). A pH 5-10 gradient is used (Fig. 2) and each sample is screened using a serum volume which contains 300 micrograms immunoglobulin G (IgG). Individuals homozygous or heterozygous for C/F are distinguished from normal unaffected individuals on the basis of the presence of a C/F factor protein band (Table 1). Heterozygous carriers for C/F are distinguished from C/F homozygotes 75 percent of the time, on the basis of a deletion in either band B, C, or D (Table 2). On the basis of screening 65 patients with cystic fibrosis, 61 heterozygous carriers for C/F, and 105 normal control subjects, it was concluded that no obvious correlation existed between either sex, age, or severity of the disease in the individual C/F patient, and the absolute presence or absence of the C/F factor. In addition, no correlation existed between sex or age and the presence of the C/F factor or deletions in proteins B, C, and D in the individual heterozygous carrier for C/F or normal control subjects. Analysis of serum samples from 68 patients with a variety of other diseases, many with clinical symptoms resembling those seen in the patient with cystic fibrosis (Table 3), indicated that the C/F factor protein described in this study appears to be diagnostic for C/F genotypes, with the possible exception of patients with certain types of leukemia.