President Bush yesterday signed a food labeling bill but declined to sign a measure designed to boost development of drugs that treat rare diseases and dilute monopoly opportunities for drug firms.

In a statement, Bush said he had "serious concerns" that the measure would remove incentives, the opposite effect of its purpose, for drug companies to develop new treatments. By withholding his signature when Congress is not in session, he killed it in what is called a pocket veto.

But Bush approved a bill requiring food manufacturers to put more nutrition information on package labels.

The labeling bill, designed to help consumers choose more healthful diets, sets standards for what information must be on food labels.

It also prohibits manufacturers from making health claims on their packaging unless scientifically proven according to the Food and Drug Administration.

The vetoed drug legislation would have amended the 1983 Orphan Drug Act giving pharmaceutical companies incentives to develop and manufacture medications generally used against diseases that affect fewer than 200,000 people.

The amendments would have watered down the exclusive rights to market the drugs for seven years that the Orphan Drug Act grants drug manufacturers.

"I believe we must not endanger the success of this program, which is due in large measure to the existence of the 'market exclusivity' provision in the Orphan Drug Act," Bush said.

The White House contends that taking away that benefit would decrease the willingness of firms to develop orphan drugs, because of the expense involved in the research and development.

But supporters of the vetoed legislation contend some drug manufacturers have misused its provisions, particularly in regard to drugs used to treat AIDS and associated infections. They say drug firms have used the law to keep drug costs artificially high.

The FDA has granted orphan status to more than 375 drugs. Critics have been concerned that some drugs were producing monopolistic windfalls for the drug companies that marketed them.

Some of the diseases that orphan drugs have been developed to treat include Huntington's disease, Tourette's syndrome, narcolepsy and muscular dystrophy.