Updated

The NFL Players Association is not ready for blood samples to be collected for human growth hormone testing, it said in a statement Wednesday.

The sides had agreed to blood testing for the performance-enhancing substance as part of the new collective bargaining agreement struck this summer.

Last Friday, after officials for the NFL and NFLPA met with members of Congress to discuss the league's plan to start testing for HGH, the league said that it was prepared to implement testing within a few weeks.

But after the league notified the union that it was ready to start collecting blood samples Monday, the union balked.

"We informed the NFL yesterday that absent a collective agreement on several critical issues, blood collection is not ready to be implemented on Monday," the NFLPA's statement said. "We have advised the players."

The union only agreed to testing once a reliable method was developed, but did not make a specific reference to that in its short statement.

In a series of twitter messages posted Wednesday, NFL spokesperson Greg Aiello said the league is "disappointed" in the union's response. Aiello said it is contrary to the terms of the CBA and agreements reached last Friday with the House Committee on Oversight and Government Reform.

Aiello said the league is ready to start educating players on the testing program and collecting samples.

"We know of no reason why these initial steps should not begin next week, and none has been identified by the union," Aiello said.

He said that approach balances immediate deterrence with the union's desire for further review.