WAUKEGAN, Ill. (AP) _ A woman pleaded guilty to killing her infant son by feeding him sulfuric acid and will testify that the boy's father planned the poisoning to collect damages from a baby-formula maker, prosecutors said.

Sheila Smith, 27, said ''guilty'' in a soft voice Friday in making her plea to a first-degree murder charge in a convoluted case that has gone before the U.S. Supreme Court.

Smith and Ricky Irby Sr., 29, were charged with murder in the death of their son Quinten Irby. Irby's trial is scheduled to begin April 30.

Quinten was 3 months old when he was poisoned in May 1984. He lingered in a hospital for more than two years before dying Aug. 26, 1986, at the age of 2 1/2 .

During Friday's hearing, Lake County prosecutor Matthew Chancey said Smith will testify that Irby told her on May 30, 1984, ''that he had a way to make some money, but it would involve hurting Quinten.''

At the time, Smith was on public aid, living with her parents in the town of North Chicago, and Irby was an unemployed auto worker.

Later that evening, Chancey continued, ''Ricky Irby told Smith that he had messed with the milk. He mentioned acid.'' The next day, Quinten was hospitalized, and Irby produced an open can of formula and a bottle that proved to be tainted with sulfuric acid.

Quinten's burns scarred his mouth and trachea. He had one lung removed, and remained on a respirator, apparently comatose, until his death 27 months later. Ingestion of sufuric acid was ruled to be the cause.

After the boy died, Irby sued a baby formula manufacturer, but the suit was withdrawn before it went to trial.

Under her plea bargain, Smith will escape a possible death penalty but could get up to 65 years in prison. Her sentencing is scheduled for June 29.

A dispute over the handling of evidence in the case at one point led to the U.S. Supreme Court.

The evidence was tested by toxicologist Joerg Pirl at a state crime laboratory in Chicago. But it and Pirl's case file were discovered missing in January 1985, well before the child died.

At one point, Pirl underwent hypnosis in an effort to find the evidence, but the hypnotist later died, and Pirl could not recall what he had told the man, said Laureen Cahill Casey, a Lake County public defender representing Smith.

Investigators later determined that the evidence had been destroyed accidentally by a lab clerical worker, and with no evidence, the case essentially was closed, Chancey said.

It was reopened after a U.S. Supreme Court ruling in November 1988 that said evidence lost by accident does not preclude prosecution. Smith and Irby were charged with murdering their son in an indictment returned May 31, 1989.

Both parents have been held on $1 million bail.

The couple's older son, Ricky Irby Jr., 9, lives with Smith's mother.

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