Multiple-transgene co-integration offers a powerful means by which several transgenes can be co-expressed in mammary glands. Independent gene constructs, including bovine alpha -casein-hG-CSF, mWAP-hEPO, and CMV-EGFP, were co-injected into fertilized mouse eggs whereupon 32% (17/54) of the transgenic mice showed integration of all the three constructs. The co-expression ratio of hG-CSF and hEPO proteins in the mouse milk was up to 54% (6/11), attributable to co-integration. Maximal expression of human EPO and G-CSF was about 1 mg l(-1) and 540 mg l(-1) milk, respectively. There was an inverse relationship between transgene fragment length and integration ratio, and evidence that co-integration events are favoured above single integration events, suggesting that integration of multiple genes may be more facilitated than a single gene. The results have important practical implications for the generation of mammary gland bioreactors, multiple transgene co-integration appearing to be a useful strategy for generating animals expressing several transgenes simultaneously.