The expression of genes introduced into various mammalian cell lines was enhanced by raising the temperature of the cells to 42 degrees C for a few hours after DNA transfection. This heat treatment resulted in an up to 10-fold increase in the frequency of the cells that transiently expressed a foreign gene such as that of beta-galactosidase, whereas it had only a limited enhancing effect on the development of stable transformants. By immunotitration analysis, it was confirmed that the enhanced expression of beta-galactosidase activity correlated well with the increase of the enzyme protein. This procedure may have an applicability for augmenting the frequency of transient gene expression in many cell types.