Alfonzo, 32, was suspended for 50 games on April 30, 2008, while a member of the Giants. At the time of that suspension, Alfonzo said he did not knowingly take a performance-enhancing substance, but took medicine for bronchitis in his native Venezuela.
Manny Ramirez of the Tampa Bay Rays tested positive for a second time in April, but retired before he could receive a 100-game suspension. A third violation makes a player subject to a lifetime ban.
Alfonzo' s suspension is in effect immediately. However, Alfonzo said in a statement released through the MLB Players Association that he plans to contest the suspension.
"I am surprised by this positive test," Alfonzo said in the statement. "I learned my lesson in 2008 and have not taken any prohibited substances since then. With the union's help, I intend to fight this suspension and look forward to appearing before the arbitrator in the near future."
A Rockies spokesman said the club would not comment on the suspension of Alfonzo beyond the following statement in a press release:
"The Colorado Rockies have long been committed to eliminating the use of performance-enhancing substances from the game of baseball. We have fully supported the adoption and implementation of Major League Baseball and the Major League Baseball Players Association Joint Drug Prevention and Treatment Program. There is no place in baseball for such substances, and we have and will continue to do what we can to eliminate them from our game."
Alfonzo began the year with Triple-A Colorado Springs before debuting with the Rockies on July 1. He has hit .267 with one home run and nine RBIs in 75 at-bats over 25 games in the big leagues this season.
Alfonzo last played Tuesday, going 1-for-1 in the Rockies' 2-1, 11-inning loss to the Brewers. Much of the playing time in recent days has gone away from Alfonzo and Chris Iannetta, the starter for much fo the season. The Rockies are taking a long look at top prospect Wilin Rosario, who was called up from Double-A Tulsa at the end of that team's season.
The suspension meant the Rockies lost two catchers on Wednesday. The Pirates announced that they claimed catcher Matt Pagnozzi from Colorado earlier in the day. Pagnozzi was with the Rockies briefly, but spent most of the year at Colorado Springs.
Thomas Harding is a reporter for MLB.com. Read his blog, Hardball in the Rockies, and follow him on Twitter @harding_at_mlb. This story was not subject to the approval of Major League Baseball or its clubs.