The expression of a recombinant human lactoferrin is reported in mouse HC11 mammary epithelial cells. Expression of human lactoferrin (hLF) was achieved by placing its cDNA under the control of the bovine ${eta}$-casein gene. To improve the hLF expression level in a cell culture system, two artificial introns were also introduced to construct expression vectors. One intron was a hybrid-splice signal consisting of bovine ${eta}$-casein intron 1 and rabbit ${eta}$-globin intron II. The other intron was a DNA fragment spanning intron 8 of the bovine ${eta}$-casein gene. The hybrid intron moderately elevated hLF expression, whereas intron 8 alone did not express any detectable amount of hLF as judged by Northem and Western blot analyses. When the two introns were used together they contributed to a synergistic elevation of hLF expression. These data indicate that artificial introns on both sides of the hLF cDNA were necessary to increase expression of cDNA.