The National Football League Players Association's executive committee last night formally rejected what league negotiators called a $1.5 billion, five-year contract proposal and authorized a strike between the second and fourth week of the season if a settlement is not reached.

After a 35-minute conference call with NFLPA Executive Director Ed Garvey, the nine-member committee called on the owners' representatives to return to the bargaining table with proposals on what it considers key issues. Among them are guaranteed shares of future NFL revenues, substantial and immediate salary increases and protection for older players who are cut from their teams because their salaries are too high.

The committee, which previously had set an unannounced strike table of between the second and fifth week of the season, also demanded reinstatement of union player representatives who have been cut from their teams.

Last night's action by the NFLPA followed a proposal from the NFL Management Council, the league's labor negotiating arm, which contained an offer of bonuses of $10,000 a year for every year in the NFL up to a maximum of $60,000 to be paid to every NFL player within 15 days after the signing of a new collective bargaining agreement.

Garvey called the bonus proposal "a no-strike bonus . . . a yellow-dog contract," noting that the offer provides for a 25 percent reduction in the bonus for each game not played because of a strike.

Garvey said the union has filed an unfair labor practice charge over that issue and over the release of union player representatives Sam McCullum, a wide receiver for the Seattle Seahawks; Mike Kadish, a defensive tackle for the Buffalo Bills, and Herb Orvis, a defensive tackle for the Baltimore Colts.

He said players on the Seahawks were so incensed by the release of McCullum, who started in all four of Seattle's exhibition games, that they might strike Sunday's season opening game against the Cleveland Browns. "The players are extremely upset that a leader of their team has now been cut because of his union activity," Garvey said.

But John Thompson, general manager of the Seahawks, denied that McCullum was released because of his union activity. "We don't let labor relations enter in any way into our decisions on personnel," Thompson said. He said there were three other wide receivers ahead of McCullum in the lineup and that Coach Jack Patera opted to go with rookie Byron Walker from The Citadel instead of McCullum, a nine-year pro, for the fourth spot. At a news conference in Seattle, McCullum said he was cut because of "politics and not policy. I am not over the hill. I have some years left. I had more than 50 receptions each of the past four seasons and that puts me among the top 10 in the league. I hope to be playing for somebody this fall."

He said if his reinstatement was ordered, he probably would end up on the bench, but he would like a chance to show the Seahawks he still could play.

"As early as last March my union activities made me undesirable," said McCullum. He said Bill Gregory, his predecessor as player representative, told him, " 'You are a marked man. They are going to release you, no matter what you do.' "

McCullum was claimed on waivers yesterday by the Minnesota Vikings, but he said he didn't know if he would report.

At an afternoon news conference, Garvey said the union's executive committee should set a strike deadline "now or within a week from now . . . we will have to demonstrate that the players have the solidarity and the ability to shut them down."

Although there may be selected strikes this weekend, the execvutive committee endorsed no such actions. The Seahawks were understood to have been meeting last night to decide what action to take Sunday. Garvey said a league-wide strike would be put off in order to permit resumption of contract negotiations and give the fans fair warning. At a meeting in Chicago last week of the full board of player representatives from all 28 NFL teams, the executive committee was authorized to call a strike at its discretion.

The union's announcement of its rejection of the offer came four hours after Jack Donlan, the executive director of the management council, presented it to a team of union negotiators at the Crystal City Marriott.

He said the bonus proposal would continue to credit players $10,000 a year severance pay on top of their individually negotiated contracts for each year in the NFL up through the expiration of the proposed agreement in 1986. Thus, he said, a player who began his NFL career in 1977 or before could collect $60,000 now and another $40,000 upon leaving the game if he played through the 1986 season.

Aside from the bonus proposal, however, the management proposal contained few changes from the initial July 13 offer that the players rejected as "little more than a rewrite" of the old contract. It would ease movement of free agents from one team to another by reducing the compensation a team signing a free agent owes the team that lost him, and increase minimum salaries, meal allowances and payments for preseason play.

Under the current system of compensating teams who lose free agents, a team signing a player making $80,000 a year forfeits a first-round draft choice to the team who loses him. Under the management offer, a player would not command a first-round pick until his salary reached $110,000. To command two first-round picks, the salary of the free agent would have to increase from $200,000 to $250,000. Moreover, the levels would increase with each year of the free agent's NFL experience.