NEW YORK — Major League Baseball implemented random blood testing for human growth hormone in the minor leagues on Thursday, the first professional sports league in the United States to take the aggressive step against doping.

The blood testing becomes part of the Minor League Drug Prevention and Treatment Program, which commissioner Bud Selig introduced in 2001 to test for performance-enhancing drugs.

“The implementation of blood testing in the minor leagues represents a significant step in the detection of the illegal use of human growth hormone,” Selig said in a statement. “HGH testing provides an example for all of our drug policies in the future.”

Testing will be limited to players with minor-league contracts because they are not members of the players’ association, which means blood testing is not subject to collective bargaining.

“Obviously, we make a separate decision with regard to the minor-league program, but the Major League Baseball Players Association has been proactively engaged in conversations with us on the scientific and logistical issues associated with blood testing at the major league level,” said Rob Manfred, executive vice president labor relations for Major League Baseball.

The players association has long been against blood testing.

“The union’s position on HGH testing remains unchanged,” union executive director Michael Weiner said.

“When a test is available that is scientifically validated and can be administered safely and without interfering with the players’ ability to compete, it will be considered.

“We have been engaged with the Commissioners’ Office on this subject for several months, though they have not shared with us the specifics behind their decision to begin blood testing of minor leaguers,” Weiner said. “We look forward to further discussions.”

Outside experts have long questioned the union’s logic against blood testing. Told of baseball’s announcement, Gary Wadler, who chairs the committee that determines the World Anti-Doping Agency’s banned-substances list, said it was “a significant step forward.”

“One important thing is, as young players evolve through the minor leagues, the concept of a blood test will no longer be alien to them,” Wadler said. “It will be easier to implement it in the major leagues as more players in the minor leagues recognize it makes sense.”

Travis Tygart, the CEO of the U.S. Anti-Doping Agency, also welcomed the news.

“This is another important step in the fight to return all of the playing fields in the U.S. to clean athletes,” he said. “We applaud MLB’s efforts in this regard.”

Tigers president and GM Dave Dombrowski said in an e-mail to The Associated Press: “Seems like a positive step for the game.”

Dodgers manager Joe Torre said: “Whatever has to be done to gain the full trust of the fans, we have to do what we can.”

Blood samples will be collected after games by the National Center for Drug Free Sport, the organization that currently collects urine samples in the minor leagues. The blood samples will be taken from the non-dominant arm of players who are not members of a major league team’s 40-man roster, and sent to a testing laboratory in Salt Lake City for analysis.

Dr. Gary Green, the medical director for Major League Baseball, called the testing “a major development in the detection of a substance that has previously been undetectable.”

“The combination of widespread availability and the lack of detection have led to reports of use of this drug amongst athletes,” Green said. “This is the first generation of HGH testing and Major League Baseball will continue to fund the Partnership for Clean Competition for ongoing research to refine testing procedures in this area.”

The Partnership for Clean Competition is a coalition of MLB, the NFL, USADA and the U.S. Olympic Committee that funds research for drug testing.

The NFL doesn’t currently conduct blood tests for performance-enhancing drugs. But the league has recently said it would like to begin such tests, while the union has long been against them. With the current collective-bargaining contract due to expire in March, the issue is expected to be a key point in upcoming negotiations.