To determine whether the mammary gland can be used to secrete large quantities of a bioactive heterodimeric protein into milk, we used a bovine β-casein promoter to target and express human follicle-stimulating hormone (hFSH) in the mammary gland into the milk of transgenic mice. We also identified the effects of hFSH leaked into the bloodstream. Transgenic mice produced a high level (up to 300 mIU/ml) of recombinant hFSH in the mammary gland. Human FSH was expressed in the mammary gland and brain, as determined by reverse transcriptase-polymerase chain reaction (RT-PCR) and immunohistochemistry. In vitro bioactivity was also identified by cyclic AMP (cAMP) assay. The highest activity was showed in the transgenic mice line 11. However, hFSH leaked into the bloodstream was a powerful factor in the generation of breast and ovarian tumors from the transgenic mice line 11. These results suggest that change of endogenous hormones (FSH and progesterone) may affect the morphology and blood cell counts of peripheral blood and, especially, provoke breast and ovarian tumors.