In many cell systems, electric pulses can efficiently mediate gene transfer with a high level of expression in vitro. In vivo results have been reported where decrease in efficiency was obtained. The mechanisms involved in the process are unknown. Since, in vivo, the efficiency of non-viral methods of gene transfer is generally limited by the presence of serum, we report here the effect of serum on in vitro electrically mediated chinese hamster ovary cell membrane permeabilization, viability, gene transfer and expression. The results indicate that permeabilization and gene transfer are not inhibited by serum. By acting as a protector of cell viability, serum indeed increases gene transfer and expression.