Holmes still is allowed to participate in the Raiders' practices and all preseason games.
His chances of making the team, however, took a hit.
Holmes released the following statement, which was obtained by NFL.com's Ian Rapoport:
"When I received a letter from the NFL stating that I tested positive for amphetamine, which is banned under the NFL performance enhancing substances policy during an off-season test in April, I was very surprised as I know that I did not take any banned substances," Holmes said. "I know that I had been quite ill when I was tested in early April, as I reported to the sample collector at the time my sample was given, so I thought one of the medications I was taking was responsible.
"However, to this day, even after further testing, I do not know how the substance got into my system. The drug policy rules are very strict with a zero tolerance, so after consulting with the NFLPA (NFL Players Association) and reviewing the medical documentation, I will not appeal the discipline. During training camp and preseason I will work extremely hard, and I will continue to work intensely so that I'm ready to contribute the minute I am back. I sincerely apologize to my teammates, coaches and Raiders fans for this situation."