Here we describe the production of cystic fibrosis transmembrane conductance regulator (CFTR), the product of the gene associated with cystic fibrosis, in the milk of transgenic mice. Mammary specific expression was achieved by placing the CFTR cDNA under the control of the goat beta-casein gene promoter. By fractionation, CFTR was shown to be associated with the membranes that envelop milk fat globules as they are discharged from the apical surface of the mammary epithelia. Since milk fat globules may comprise up to 10% of whole milk, this represents a novel, inexpensive and efficient approach to produce CFTR and possibly other membrane-associated proteins. The availability of large quantities of CFTR could have important implications for the development of new therapies for cystic fibrosis.