THERE'S BEEN A PAINFUL separation at one of Washington's leading personal injury law firms. After 19 years with Ashcraft, Gerel and Koonz, senior partner Joseph H. Koonz Jr. moved out last month angry and hurt. When he left, he took two young associates, his secretary and his office furniture, and started a new law firm.

Successful law firms are built on strong personalities and loyalties, and it's unusual that they are set so firmly against each other that a firm is driven apart. But that's what happened when Joe Koonz faced off with the partners at Ashcraft, Gerel. There followed a bitter, at times emotional dispute complete with a $500,000 lawsuit (now being settled), accusations made and then withdrawn, pleas for reconciliation that failed and finally a truce, with Koonz deciding to get out.

Amidst all this were hints of a power struggle between Koonz and the partners. The most that each side will say about what went wrong is that Koonz and the firm split because of a dispute over advertising.

"Little things happened, but the thing that really got to me was the ads. I didn't believe then and I can't believe now that competent lawyers who are doing their job had to resort to an ad," Koonz said. The firm, which specializes in workers' compensation cases, had become successful by reputation, Koonz said during an interview. "We weren't selling cars."

"The firm is not one man's playpen," said Martin E. Gerel, one of the founding partners of the firm.

"I'm not suggesting this is what Mr. Koonz had in mind," he said during an interview, adding, "we're a democratic firm. We vote and however the majority votes, we go."

The vote that helped lead to Joe Koonz's decision to leave the firm occurred about a year ago.

It was the partners' 6-to-1 decision to advertise in the Washington Yellow Pages. Koonz was the lone dissenter.

"I said, it's greed, absolute greed, I told them to their faces they were greedy. There was no other explanation," Koonz said.

Gerel said he remembered Koonz using the word "greed," but he strongly disagrees that was the motivation behind the decision to advertise.

"Ours is a public practice. We don't represent two or three large corporations. If our clients were General Motors, Ford and Chrysler, it would be another story. But since the public is our client, we thought it would be in their interest as ours to advertise," Gerel said.

It was agreed the ad, without Koonz's name, would run for a year, and only in Washington.

Last spring, however, at the Harvard Club in Boston, senior partner Lee C. Ashcraft told him that the firm had decided to go ahead and advertise in the Maryland and Virginia Yellow Pages. Koonz described himself as "hurt" and "disgusted." Ashcraft, in a telephone interview, said he didn't remember that conversation.

The next Tuesday, Kootz said, he asked for a meeting of the partners. On Wednesday night Gerel and partner James A. Mannino showed up at his home in suburban Maryland Koonz said.

"They said, "Don't lose your head," Koonz said. But by then, Koonz said, "I decided it was time for me to go on my own." He told the partners he intended to leave in July 1980. They were upset and an attempt at reconciliation began.

"What will it take to keep you here?" Koonz said Gerel asked him. Koonz responded with a lengthy memo. According to Ashcraft, Koonz wanted to direct a decision-making committee for five years and appoint the members. The terms of Koonz's proposal, however, were "completely unacceptable," Ashcraft said. It was then that the real trouble began.

A $500,000 lawsuit was eventually filed in D.C. Superior Court by the partners of Ashcraft and Gerel against Koonz and the two associates who also were leaving, Roger Johnson and Carolyn McKenney Endless. The firm, which now says it doesn't intend to pursue the case, charged that Koonz planned to take files, clients and fees with him to his new firm, in violation of a partnership agreement. It said Koonz encouraged associates to leave Ashcraft and Gerel and come with him, all of which "created an unsettled and disruptive atmosphere" within the firm.

Endless and Johnson, in their response, agreed that they told partner Mannino, on Thursday, Sept. 6, that they intended to go with Koonz.