Recent physiological studies have shown a defective beta-adrenergic regulation of chloride transport and protein secretion in tissues affected by cystic fibrosis. The exact biochemical nature of this abnormality is unknown, but an intracellular second messenger may be involved. We have tested the hypothesis that calmodulin is the site of the basic defect in CF using biochemical and molecular genetic techniques. We report here that there is no gross structural abnormality in the calmodulin protein from CF submandibular glands, and that although there are at least three distinct sequences that cross-hybridise with a calmodulin cDNA probe in the human genome, none of these can be the locus of CF. A polymorphism at the locus of a calmodulin cross-hybridising sequence at human chromosome 7p2 is described.