WASHINGTON (AP) _ The Supreme Court today refused to kill a lawsuit against the owner of a Missouri pharmaceutical processing plant accused of endangering health by sending low-level radiation into the air.

The court, by a 6-3 vote, let stand a ruling that federal regulation of nuclear safety does not bar a state court lawsuit against Mallinckrodt Inc.

Justices Byron R. White, Thurgood Marshall and Sandra Day O'Connor voted to hear arguments in the case, but four votes are needed to grant such review.

The plant in Maryland Heights, Mo., produces what are known as radiopharmaceu ticals that are injected into patients for diagnosis and treatment of diseases.

Mallinckrodt was sued for $1 billion on behalf of more than 700 employees of other businesses and factories in the area who say they are being exposed to harmful radiation.

A state judge in Missouri threw out the suit, but it was reinstated last July by a state appeals court.

The appeals court rejected arguments by Mallinckrodt that it should be immune from a state lawsuit because it is complying with federal regulations on radiation emissions.

The appeals court said, ''Mallinckrodt is not guaranteed absolute insulation from the consequences of its acts through compliance with federal regulation. Mallinckrodt can continue to meet federal standards and, if found wanting by state standards, simply pay the piper.''

The appeals court based its reasoning in part on a 1984 Supreme Court decision that opened the way for reinstatement of a $10 million negligence award for the three children of the late Karen Silkwood.

Silkwood, a laboratory analyst at an Oklahoma plutonium plant, was contaminated with radiation that escaped from the plant.

Lawyers for Mallinckrodt said the Missouri appeals court made a mistake in relying on the Silkwood ruling.

They said the Supreme Court ruling applies only to lawsuits ''that result from a significant accidental release of radiation into the surrounding environment.''

In the case of Mallinckrodt, the amounts of radiation are low and are released into the atmosphere deliberately as a necessary part of the company's federally authorized operations, its lawyers said.

They added that allowing suits based on such emissions ''ignores clear expressions of congressional intent to preclude such intrusive state regulation of nuclear safety.''

The company was supported by the American College of Radiology and the Society of Nuclear Medicine.

Those groups said the Missouri court's ruling, if adopted by other courts, could undermine important new medical advances.

Makers of radionuclides used to detect and treat disease could be forced to abandon production if faced by costly lawsuits, the groups said.

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Eds: This case is Mallinckrodt vs. Bennett, 85-1710.