A series of cystic fibrosis homozygous fibroblast X mouse cell hybrids were constructed in order to determine the chromosome which carries the gene controlling the cystic fibrosis mucociliary inhibitor (CFMI) phenotype. Several hybrids and their subclones retained the CFMI phenotype for long periods in cell culture. Correlation of the CFMI phenotype with the presence of specific human chromosomes indicated that possible linkage existed between CFMI and human chromosomes 2, 4, 6, 10, and 18. The strongest chance of linkage existed for chromosome 4.