Synthetic biology is a relatively new science with tremendous potential to change how we view and know the life sciences, but like many developing technologies, it has provoked ethical concerns from the scientific community and the public and confronts demands for new regulatory measures. The concerns raised involve the danger of "dual use," in which results for improving human well-being and the environment may be misappropriated for bioterror.To counteract these dangers, many governments, but the United States and Israel in particular, have introduced new laws and redoubled measures for biosafety and biosecurity. In the United States, the recent H5N1 results achieved by two groups of NIH-funded investigators highlighted the dilemma of balancing the risk of dual-use research and the freedom of science. In Israel, concern for unconventional terrorism is long-standing, and the country is constantly engaged in improving biosecurity and biodefense measures. In 2008, the Israeli parliament passed the Regulation of Research into Biological Disease Agents Law, a legislative framework for safeguarding research into biological disease agents.This article summarizes and analyzes the current state of affairs in the United States and Israel, ethical attitudes, and regulatory responses to synthetic biology.