NEW HAVEN, Conn. (AP)--Yale University has given striking graduate students until noon tomorrow to turn in undergraduates' fall semester grades or face losing their teaching appointments for the next semester.

The deadline came in a letter to part-time acting instructors--graduate students who run their own classes.

"We undertake this course of action with great regret," the letter said. "The time has come when we must address the deliberate refusal of some graduate students."

The letter was signed by Yale President Richard C. Levin, Provost Alison Richard, Graduate School Dean Thomas Appelquist and undergraduate Yale College Dean Richard H. Brodhead.

The graduate students, who teach the humanities and social sciences, have instituted the "grade strike" in a bid to be recognized as a union by Yale.

They want higher teaching stipends, lower health-insurance costs, smaller classes, more teacher training and a procedure to air grievances.

The group, called the Graduate Employees and Students Organization, has said grades will be withheld until Yale's administration agrees to negotiate for a contract. No talks are scheduled.

The original deadline for the grades was Jan. . The missing grades weren't an immediate crisis for undergraduates. They are on winter break, with classes not scheduled to resume until Jan. 15. They typically get their grades then and most parents receive the marks about two weeks later.

The teaching group represents about 25 percent of the total 2,500 graduate students. Yale maintains that they don't have the right to strike because they are students, not employees.

The letter was aimed at 29 out of a total 135 part-time acting instructors. Thus far, those 29 have failed to turn in approximately 435 grades to the university registrar, said Yale spokesperson Gary Fryer.

However, it was unclear whether the missing grades were due solely to the grade strike or other circumstances, Fryer said.

Robin Brown, group chair and a part-time acting instructor, said Friday she will continue to withhold the grades for her first-year English course.

"It's a serious threat. That's the money I use to live on," Brown said, "We're beginning to prepare for the worst. Our plans are to fight it."

A similar letter will be sent in the coming days to graduate students who act as teaching assistants for classes run by professors, administrators said, Many of those 304 graduate students teach discussion sections in Large lecture classes.

The university was still canvassing professors Friday to determine how many of the assistants had withheld grades. The Tuesday deadline does not apply to them, although a deadline will be given to them, Fryer said.

The group claims about 240 graduate students have refused to turn in marks. Union organizer and former Yale graduate student Gordon Lafer said the graduate students stand firm despite the threat of losing their jobs.

"Their tactics have been to talk tough to try to scare people, but I don't think this is going to have any affect on the strike," he said. "This just shows a lot of what's wrong here and why people want a union."