Transgenesis has been used for expressing human lysozyme (hLZ) in the milk of livestock to improve their disease resistance. Here we describe a human lactoferrin (hLF) BAC as a candidate vector for high-level expression of hLZ in the milk of transgenic mice. Using recombineering, hLF genomic DNA in the hLF BAC was replaced by the hLZ gene (from the ATG start codon to the TAA stop codon), and flanking regions of the hLF gene (a 90-kb 5' and a 30-kb 3') were used as transcriptional control elements for hLZ expression. When this construct was used to generate transgenic mice, rhLZ was highly expressed in the milk of four transgenic mouse lines (1.20-1.76 g/L), was expressed at a lower level in one additional line (0.21 g/L). rhLZ from the milk of these transgenic mice exhibited the same antibacterial activity as native hLZ. Our results suggest a potential approach for producing large amounts of hLZ in the milk of livestock.