WILMINGTON, Del. (AP) _ A teen-ager pleaded guilty today to manslaughter and agreed to testify against his girlfriend for the death of their newborn son, who was found wrapped in plastic in a motel trash bin.

In exchange, prosecutors agreed to drop murder charges against Brian Peterson, which would have carried a sentence of death, or life in prison. Instead, Peterson could receive up to 10 years in prison for manslaughter.

``Brian has explained at great length the infant didn't show any signs of life,'' said his attorney, Russell Gioiella. ``He believed the baby was dead. ... Brian had no intent to harm the baby in any way.''

Meanwhile, Gioiella said, his former high school sweetheart Amy Grossberg was saying, ``Get rid of it. Get rid of it.''

The two 19-year-olds from a wealthy northern New Jersey suburb were both accused of murdering their son in November 1996 in a motel near the University of Delaware, where Ms. Grossberg was a freshman art student. Peterson was a freshman at Gettysburg College in Pennsylvania.

Ms. Grossberg has pleaded not guilty to first-degree murder and murder by abuse or neglect charges.

Her attorneys had already been distancing themselves from Peterson, requesting separate trials and saying she believed the child was stillborn and played no role in disposing of the body.

The baby was found wrapped in plastic in a trash bin outside the motel. A medical examiner said the child suffered severe head trauma.

Both teens have been living with their families in Wyckoff, N.J., free on $300,000 bail but monitored electronically.

Ms. Grossberg was not in court today. During a hearing Friday, the two avoided eye contact and did not speak to each other.

A sentencing date was not immediately set for Peterson.