MUC-1 mucin is an epithelial cell antigen whose aberrant expression plays a role in autoimmunity and tumor immunity and is thus an attractive candidate for immunotherapy of gene therapy. Because the MUC-1 cDNA is composed almost entirely of 60-bp tandem repeats and is susceptible to homologous recombination, it presents a special challenge to cloning and expression in viral vectors. Nevertheless, we have been successful in constructing a retroviral vector (MFG-MUC-1) with a 22-tandem repeat MUC-1 cDNA. Both stable and transient packaging cell lines are capable of producing high-titer retroviruses that can transfer the expression of MUC-1 to murine 3T3 cells. Transduced cells express uniformly high levels of MUC-1 on their surface, and western blot analysis reveals that the molecule expressed is of full length and extensively glycosylated. We have used the MFG-MUC-1 vector to stably transduce an immortalized murine dendritic cell line and show that immunization of mice with transduced cells elicits specific immune responses to mucin. The ability of this vector to transfer expression of the MUC-1 tumor antigen to potent antigen-presenting cells is expected to be of use in the immunotherapy of epithelial cancers.