Major League Baseball’s players union told agents in an e-mail message on Friday that the trading card company Upper Deck was experiencing financial difficulty and had not paid a large number of players money it owed them.

The union said it was working with Upper Deck to ensure that the players would be paid. The union strongly advised players not to return to Upper Deck any autographs they had signed for the company or to enter into any new agreements with the company until the payments were made.

The e-mail message was from Evan Kaplan, the union’s director for trading cards and collectibles, and was sent to all player agents. A person who received the e-mail provided a copy of it to The New York Times on the condition that the person not be identified.

A spokesman for the union declined to comment. A spokesman for Upper Deck did not respond to an e-mail message.