Free agency is the only real issue that may cause a strike next week in the National Basketball Association. The owners want to end it and the players want it preserved.

The players, who two months ago set an April 2 strike deadline on a new contract, walked out of the last negotiating session Thursday after only 20 minutes.

Larry Fleisher, players association general counsel, said yesterday that a strike is likely.

He said that after 12 hours of meetings Monday and Tuesday, he was very optimistic, and even called all of the player representatives and said he thought an agreement was near. But after only 20 minutes Thursday, he said he was disillusioned and the talks broke off. No new talks are scheduled.

"We don't want a strike," Fleisher said. "It wouldn't be good for anybody, but we have to protect our rights. When somebody sets a fire at your door and you throw water on it, you shouldn't become the bad guy just for getting things wet."

In place of the current free agent system set up by the Oscar Robertson settlement in 1976, the owners have proposed what they call a guaranteed compensation plan that would set maximum and minimum team salaries and guarantee the players a fixed percentage of the gross revenues.

The players, paid approximately 53.8 percent of the gross this season, had asked for 57 percent of the gross. The owners offered 40 percent.

As of last Thursday's meeting, the owners offered 50 percent and the players said they'd settle for 53.

That's still only half the issue, however, because the free agency problem remains. The players originally agreed to the guaranteed compensation plan with the percentage, the salary cap and minimum to be negotiated. But they didn't want it started until after the 1986-87 season when the Robertson settlement expires.

The NBA has made no public statements since early last week and declined to do so again yesterday.

Under terms of the guaranteed compensation plan, the league originally proposed a salary cap of $4 million and a team minimum of $2 million. Those numbers, through the negotiating sessions last Monday and Tuesday, were increased to a mimimum of about $3.2 minimum with a cap above $4 million.

A team with a payroll above the cap wouldn't be allowed to sign any free agents, but those under the minimum would be required to increase their payroll, thus forcing more parity, as well as still allowing player movement.

The Washington Bullets, for example, with a payroll of about $2 million a year, would, therefore, have to spend another $1.2 million to get to the minimum.

Fleisher said, however, that at Thursday's meeting, the owners reversed their position and wanted to set the cap at $2.8 million and the minimum at $1.6 million.

Fleisher said that since only a couple of teams have a payroll of $1.6 million or less and most are over $2.8 million, the ones over can't bid on free agents and the ones who aren't over the maximum won't have to. Thus free agency would be ended.

"And we aren't going to let that happen," said Fleisher.

An NBA source said that the percentage of gross revenues would be the basis for the salary maximum. To reach the maximum, the league would divide its revenue from the previous year by the agreed upon percentage and then divide that figure by 23, the number of teams in the league.

The basis of the current dispute started 13 years ago when the league had only 14 teams and the average salary was less than $50,000 a year. In 1970, a class action suit with Oscar Robertson's name listed first, thereby labeling it the Robertson suit, was filed, charging the NBA owners with unfair restraint of player freedom.

When a bidding war between the NBA and the now defunct American Basketball Association developed in the early 1970s, there was talk of a merger. A federal judge handed down an injunction prohibiting the NBA from discussing a merger with the ABA until after the Robertson suit was resolved.

An out-of-court settlement was reached in 1976, setting up the current system of free agency.

Under the Robertson settlement, which went into effect with the 1976-77 season, option clauses were no longer attached to contracts.