Matrix attachment regions (MARs) have been shown to participate in the insulation of transcription elements from surrounding chromatin in tissue culture cells and transgenic mice. A whey acidic protein (WAP) transgene containing 1 kb promoter sequence was active in mammary tissue from 1 out of 17 lines of mice, demonstrating that the transcription elements were highly susceptible to position effects. To test whether MARs could insulate this WAP gene promoter and thereby restore transcription, we ligated MARs from the chicken lysozyme gene to the WAP transgene. Seven of the nine lines generated exhibited WAP transgene activity, expression was confined to mammary tissue, and correct regulation was observed in three of the four lines analyzed. This study provides strong additional evidence that the MAR fragments from the chicken lysozyme gene have the capacity to insulate transgenes from severe position effects.