Rienzo, a Carolina League All-Star last summer with Class A Advanced Winston-Salem, and Guerrero, a left-hander who last played for the Lake County Captains, both tested positive for Stanozolol, a performance-enhancing substance.
The suspensions of both players are effective immediately.
Rienzo, a 6-foot-3 right-hander from Sao Paolo, Brazil, is 3-0 with a 1.08 ERA and 31 strikeouts in 25 with the Dash. He signed with the Chicago White Sox as a free agent in 2006.
Guerrero, 21, signed with Cleveland in 2008 and spent the 2011 season with Class A Lake County and Class A Short-Season Mahoning Valley, where he went 2-4 with a 5.74 ERA in 18 outings. He has yet to appear in a Minor League game this season.
Stanozolol is a synthetic anabolic steroid derived from testosterone that is sometimes prescribed by veterinarians to encourage muscle growth, red blood cell production, bone density and to stimulate the appetite of weakened animals. It's also been linked to body builders and several former baseball players.
The Office of the Commissioner of Baseball has now suspended 37 players in 2012 for their violations of the Minor League Drug Prevention and Treatment Program.