In 1991, Congress enacted the research and experimentation tax credit to encourage businesses to do research. GAO estimates that the pharmaceutical industry earned $1.24 billion worth of these tax credits between 1981 and 1990. The industry's credits as a share of the credits earned by all industries rose from four percent in 1981 to 12 percent in 1990. These credits were earned primarily by large companies--those with assets of $250 million or more. The biotechnology sector benefitted very little from the tax credit. The research and experimentation tax credit is difficult for the Internal Revenue Service (IRS) to administer because (1) auditors have to distinguish stages of research, (2) audit issues are often highly technical, and (3) successive proposed regulations cause uncertainty.