King tested positive for Ritalinic Acid, and Contreras tested positive for metabolites of Nandrolone. Each will serve their suspensions at the start of next season.
King, 21, hit a combined .226 with seven home runs and 35 RBIs in 94 games for Class A Hagerstown and Class A Potomac this season. Contreras, 17, his .205 with a homer and six RBIs in 29 games for the Cubs' Dominican Summer League team.
Alden Gonzalez is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.