It will apply on disputed home run calls.

The umpires had balked earlier this week when asked to become a part of the process.

The 30 major league ballparks are being wired with monitors so umpires on location can view replays of questionable home run calls -- fair or foul, in the park or out, and many installations are done, but some are being completed.

"We're not there yet," MLB's senior vice president of public relations, Rich Levin, told MLB.com after the agreement was reached Wednesday. "(But) this is another significant step."

"I told our members that it's just another tool that we can use to get the play right, not to look at it as a negative, but a positive," said WUA president John Hirschbeck.

The use of instant replay could start later this month.