We describe the generation of transgenic mouse lines expressing Cre recombinase in epithelial cells of the lactating mammary gland. As an expression vector, we used a P1‐derived bacterial artificial chromosome (PAC) which harbors the gene for the secretory milk protein, whey acidic protein (Wap). Using homologous recombination in E. coli, the PAC was modified to carry the improved coding sequence of Cre recombinase (iCre). Transgenic lines carrying the WAPiCre PAC express Cre recombinase efficiently in the majority of mammary epithelial cells upon lactation. Of only four transgenic lines produced, three express Cre recombinase to a high efficiency. LoxP‐flanked DNA sequences are recombined in virtually all epithelial cells of WAPiCre transgenic mice at lactation day 3. genesis 33:125–130, 2002. © 2002 Wiley‐Liss, Inc.