Pgk-1 is an X-linked gene encoding 3-phosphoglycerate kinase, an enzyme necessary in every cell for glycolysis. The regulatory sequences of the Pgk-1 gene were used to drive the E. coli lacZ reporter gene and 2 strains of transgenic animals created with this Pgk-lacZ transgene carried on autosomes. The levels of expression of Pgk-1 varied from one adult tissue to another and the transgene was similarly regulated. However, in situ staining of the beta-galactosidase encoded by the transgene indicated extensive cell-to-cell variability in its level of expression. A reproducible subset of cells stained darkly for the transgene product. Some of these beta-galactosidase positive cells were rapidly proliferating while others appeared to be metabolically very active, suggesting that the Pgk-1 promoter is regulated so as to be more active in cells requiring high levels of glycolysis. Although Pgk-1 is X-linked and subject to X chromosome inactivation, the transgenes were not inactivated in either female somatic or male germ cells. Thus, the Pgk-1 promoter drives transgene expression in all tissues but the levels of expression are not uniform in each cell.