We transfected a mouse myeloma cell line, P3/NS1-Ag4-1 (NS-1), and a chinese hamster ovary cell line, CHO-K1, with the β-galactosidase (β-Gal) gene of Escherichia coli, and isolated stable transformants, designated as NS-1Z/gpt and CHO-Z/neo, respectively. When these cells were incubated with 5-10 mM 2-aminopurine (2-AP) for 12-24 h, the specific and the total activity of β-Gal was enhanced 2- to 4-fold and 1.5- to 2-fold, respectively. It was confirmed in immunotitration experiments that the enhancement of β-Gal activity was due to the increase of the enzyme protein. Northern blot analysis revealed that 2-AP augmented the expression of β-Gal mRNA. On the other hand, 2-AP did not significantly affect the expression of an endogenous gene like lactate dehydrogenase or β-actin.