Transgenic mice were produced to study the production of bovine alpha-LA in their milk. A 7.6-kb fragment containing a bovine alpha-LA gene was purified and microinjected into pronuclear stage mouse embryos. This fragment contained 2.0 kb of 5' flanking region, the 1.7-kb coding region, and 2.7 kb of 3' flanking region. Out of 121 potential transgenic founder mice, 3 were identified as being transgenic by the polymerase chain reaction. Multiple mice from the second, third, and fourth generation from each line were milked, and the milk was analyzed using an ELISA assay and Western blots to determine the presence of bovine alpha-LA. Bovine alpha-LA was present at concentrations up to 1.5 mg of protein/ml of mouse milk. The high degree of expression variation between mice within each of the transgenic lines was a characteristic that has not been reported in other studies of transgene expression in milk. Production of bovine alpha-LA in the milk of these transgenic mice showed a high degree of variation both within a lactation and between mice within a line. The bovine alpha-LA concentration in a single line of transgenic mice exhibited as much as a 10-fold variation between mice. Variations as high as 3-fold were detected within a single lactation in the same mouse. These differences in expression appeared to be correlated with mouse milk production; bovine alpha-LA was higher on d 10 and 15 of lactation than on d 5. Transgenic mice that show variation in expression of a bovine gene might offer a unique system for studying quantitative traits in a laboratory model.