Washington ? For the first time since October, ailing Chief Justice William H. Rehnquist presided at a public session of the Supreme Court. Though his voice was thin and faint at times, he took a lively part in the back-and-forth arguments that stretched over two hours Monday.

“I want to extend to each of you a warm welcome as a member of the bar of the court and as an officer of the court,” the chief justice told a dozen lawyers who took an oath as members of the Supreme Court bar.

The 80-year-old Rehnquist had a tracheotomy as part of his treatment for thyroid cancer, and he spent more than a month at home recovering. But since January, the chief justice has come to the court regularly and participated in deciding the court’s cases.

By coincidence, the first case heard Monday tested a key opinion written by Rehnquist in 1989. In the case of Joshua DeShaney, who was severely injured in a beating by his father, the chief justice had said public agencies — in that case, a Wisconsin child welfare office — could not be sued in federal court for failing to provide certain services. The Constitution protects people against actions by public officials, but it does not guarantee them any services, he wrote.

With the chief justice back in the center seat, lawyers for Castle Rock, Colo., invoked Rehnquist’s 1989 opinion to say their police department could not be held liable in federal court for failing to come to the aid of a woman who had a protective order against her estranged husband.

The husband later killed the couple’s three daughters.