Northrop Grumman Corp., in another stark illustration of the human cost of spending cutbacks in the defense industry, said Thursday it will slash 8,650 jobs during the next 15 months, including 4,150 in Southern California.

The job losses are "a painful but necessary step to reduce our costs" and to make the contractor "a leaner, streamlined company" in the face of the defense slowdown, Northrop Grumman Chairman Kent Kresa said in a statement.

The Los Angeles-based aerospace company said it does not yet know how many of the cuts will involve actual layoffs. The number will depend on attrition and how many workers accept an early retirement program being offered to 5,000 employees.

If 60% of the eligible workers accept its early retirement offer, Northrop Grumman said it expects to incur a one-time, pretax charge against earnings of about $300 million in this year's fourth quarter.

"We're sorry to see this happen," said Tom Quintana, a spokesman for the city of Hawthorne, one of the sites hard hit by Northrop Grumman's move. "If there's a ray of good news, at least some of the employees will have the option of getting retirement (benefits) out of it."

Some job losses were expected following Northrop's $2.2-billion purchase of Grumman Corp. in May and its acquisition last month of the 51% of Vought Aircraft Co. in Texas that it didn't already own.

But Northrop Grumman said only about 1,000 of the cuts--which will also be concentrated in New York and Texas--are directly related to those deals. The bulk of the lost jobs are due to the relentless slide in Pentagon spending, which continues to leave Northrop Grumman and other defense contractors with far more capacity than they need.

While the Northrop Grumman cuts were anticipated, the company had not previously indicated how big they would be. They turned out to be substantial--about 18% of its current worldwide employment of 47,500.

The reductions include:

* 2,400 jobs at Palmdale and Pico Rivera, where the B-2 Stealth bomber is built. Northrop Grumman first disclosed these cuts last week. By 1996, the remaining 8,000 jobs at these sites will also be in jeopardy, because the B-2 program is scheduled to be completed by the decade's end.

* 1,750 at Hawthorne and El Segundo, where Northrop Grumman makes fuselages for the McDonnell Douglas F/A-18 fighter jet as well as an attack missile known as TSSAM, aircraft sensors and other defense electronics. The two sites currently employ about 10,000 workers.