Talks over pay and other conditions between National Football League team owners and players have broken down, threatening the 2011 season.
Owners imposed a "lockout" at midnight (0500 GMT Saturday) after an agreement with the players' union expired.
The move marks the first NFL labour stoppage since 1987, when replacement players were used during a strike.
Top players Drew Brees, Peyton Manning and Tom Brady have filed antitrust lawsuits against the league.
The new season is due to start in September.
The players' association, the NFLPA, has "decertified", in effect renouncing its status as a union and meaning arrangements made in the past between the union and the NFL are no longer immune to competition law.
The main sticking point was how to distribute the league's $9bn (£5.6bn) in annual revenues.
Players for the NFL's 32 teams currently receive 60% of the revenues, something the clubs' owners see as too generous.
The NFL takes $1.3bn for costs but it says these have risen steeply since the previous deal was struck in 2006 and it now wants to take $2.3bn, which would mean less money for the players.
The talks also covered wages, drug testing, pension benefits, and plans to increase the number of regular-season games from 16 to 18.