Cyclist Lance Armstrong denied allegations that he participated in banned performance-enhancing measures, questioning the credibility of former teammate Floyd Landis, who admitted his own use of drugs and other practices in recent emails.

Mr. Landis, the American cyclist whose 2006 Tour De France victory was nullified after a positive doping test, in recent weeks sent a series of emails to cycling officials and sponsors admitting to, and detailing, his systematic use of blood transfusions and performance-enhancing drugs during...