Members of a congressional committee asked the NFL and players’ union Friday for “immediate written assurances” that pro football’s plan to blood test players for human growth hormone will be implemented for the upcoming season.

Four members of the House committee on energy and commerce made the request in a letter Friday to NFL Commissioner Roger Goodell and DeMaurice Smith, the union’s executive director.

“It is well past time to fulfill last year’s agreement, protect the health and safety of your players and their youngest fans and guarantee the integrity of the National Football League that is beloved by millions of Americans,” the members of Congress wrote in the letter.

The letter was signed by Rep. Fred Upton (R-Mich.), the committee chairman; Rep. Henry Waxman (D-Calif.), the committee’s ranking Democrat; Rep. Mary Bono Mack (R-Calif.); and Rep. G.K. Butterfield (D-N.C.).

The league and union agreed last year as part of a new 10-year collective bargaining agreement to implement HGH blood testing and targeted the outset of last season for the start of the program. But the two sides first had to agree to testing procedures. That still hasn’t happened, and testing has not begun.

The union has raised several objections to the league’s testing plan and the two sides have been unable to resolve their differences.

One person familiar with the deliberations said recently that dialogue between the league and union has continued, with some progress but also some setbacks. It remains unclear whether testing will be in place when the 2012 season begins in September, said the person, who spoke on the condition of anonymity because of the ongoing discussions.

Greg Aiello, the NFL’s senior vice president of communications, said in a written statement issued Friday that “we appreciate and share the concern of the committee, and will respond to the letter as soon as possible.”

George Atallah, the union’s assistant executive director of external affairs, said: “We remain committed to finding a safe, fair and effective test.”