Elena Kagan begins hearing cases as the Supreme Court's 112th justice Monday morning. But anyone who wants to see her in action needs to be sharp.

Kagan will hear the first case argued before the court, then slip quietly through the burgundy velvet curtains behind the bench. She'll be out of the action in all three cases : Tuesday. Her chair will be empty when the court returns next Tuesday and she'll put in a half-day the next day.

Kagan's old job as solicitor general - the "10th justice" - is initially making it hard to do her new job as the ninth justice.

Kagan, 50, has recused herself from 25 of the 51 cases the court has accepted so far this term, all as a result of her 14-month tenure as solicitor general, the government's chief legal representative in the Supreme Court and the nation's lower appellate courts.

The recusals are one measure of how integral the "SG" is to the court's workings. Much of the court's caseload comes from challenges to federal statutes or government policies that the solicitor general must defend. The court also often asks for the government's view on whether a case is ripe for review.

Kagan is recusing herself from cases in which she had a role in drafting a brief for the Supreme Court, or when she was actively involved in a case in the lower courts. She took herself out of such deliberations when President Obama nominated her last May, so the pace of her recusals should slow as the court over the next few months completes the work of filling the term's docket.

But initially, Kagan's absence will affect some important corporate and employment- discrimination cases, as well as a highly anticipated review of one of Arizona's attempts to crack down on illegal immigrants.

The issue might affect the court in other ways. Steven R. Shapiro, legal director of the American Civil Liberties Union, said last week that some lawyers are waiting on bringing issues to the court until they can be sure Kagan could hear them.

An eight-member court creates an advantage for the party that won at the lower level: it needs to convince only four justices in order to win, because an evenly divided court keeps the lower-court ruling in place without creating a national precedent.

The immigration case, Chamber of Commerce v. Whiting, is an unusual alliance between business groups, civil liberties organizations and the federal government to overturn a law that allows the state to yank a business's license for hiring undocumented workers.

The U.S. Court of Appeals for the 9th Circuit upheld it, but the solicitor general's office told the court that it intruded on the federal government's exclusive authority on immigration laws.

Kagan's absence may be felt most strongly in two cases that ask whether federal regulation protects companies from lawsuits brought under state consumer protection laws.