Chief Justice John Roberts has sold his shares of Pfizer Inc., a move that allows him to participate in two pending Supreme Court cases involving the pharmaceutical maker.

Federal law requires judges to sit out cases if they own even a single share of stock in any of the parties to a lawsuit. In the past, Roberts has not taken part in cases involving Pfizer because he owned less than $15,000 of the company's stock, according to his latest report of personal finances, which covered 2009.

But when the court announced Tuesday that it had accepted an appeal from several drug makers, including Pfizer, in a dispute over prices charged public hospitals, there was no indication that Roberts would step down from hearing the case.

Supreme Court spokeswoman Kathy Arberg confirmed that the chief justice sold his Pfizer shares in August.

As a result, he also will sit with his colleagues when they hear arguments Oct. 12 in another case involving childhood vaccines. Roberts played no role when the court decided in March to hear the case.

Arberg offered no explanation for Roberts' decision to sell the stock now, but it appears likely that Justice Elena Kagan's need to sit out the two cases played a role in Roberts' timing.

Kagan, who joined the court in August, owns no stock in Pfizer, but was involved in the two cases at some point during her time at the Justice Department as President Barack Obama's solicitor general.