ALBUQUERQUE, N.M. —  The director of the problem-plagued Los Alamos National Laboratory has resigned, effective immediately, a University of California spokesman said Thursday.

The spokesman, Rick Malaspina, said an interim director has been appointed to replace Director John Browne. He did not immediately identify the replacement. The university runs the nuclear research lab for the federal government.

Malaspina did not give a specific reason for Browne's resignation but said it was a "mutual decision" by him and university officials. Lab employees were being informed of the change, he said.

The lab has been under investigation for alleged lapses in property and monetary controls.

A memo released by whistle blowers in November said nearly $1.3 million worth of computers, phones and other property was unaccounted for at the lab in the budget year 2001.

Last month, an audit report was released that said nearly $4.9 million in employee credit card purchases at the lab were not processed properly, were questionable or had been disputed by the lab and not credited by the bank.

Browne said at the time that the lab had already fired one employee for using a lab credit card to buy food and gas and get cash advances and was considering action against three others.