Holmes will be eligible to return to the Raiders' active roster Sept. 30, the day after they host Washington in the fourth game of the season. He will be allowed to participate in all preseason practices and games.
Holmes said in a statement that he was surprised when he found out that he tested positive for a banned amphetamine during an offseason test in April.
"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," he said. "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 (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."
Holmes was claimed off waivers from New England in May. He played seven games for Dallas last season, catching two passes for 11 yards.
Coach Dennis Allen said the team knew a suspension was a possibility but said it was a league matter and he would have no additional comment.
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