Graham Boyd, the director of the American Civil Liberties Union drug law project, said Mr. Holder’s remarks created a reasonable balance between conflicting state and federal laws and “seem to finally end the policy war over medical marijuana.” He said officials in California and the 12 other states that have authorized the use of medical marijuana had hesitated to adopt regulations to carry out their laws because of uncertainty created by the Bush administration.

Image Eric H. Holder Jr. outlined a new enforcement policy. Credit... Chip Somodevilla/Getty Images

Mr. Holder said the new approach was consistent with statements made by President Obama in the campaign and was based on an assessment of how to allocate scarce enforcement resources. He said dispensaries operating in accord with California law would not be a priority for the administration.

Mr. Holder’s comments appeared to be an effort to clarify the policy after some news reports last month interpreted his answer to a reporter’s question to be a flat assertion that all raids on marijuana growers would cease. Department officials said Mr. Holder had not intended to assert any policy change last month but was decidedly doing so on Wednesday.

Ethan Nadelmann, the founder of the Drug Policy Alliance, said Mr. Holder was telling the Drug Enforcement Administration that it should leave legitimate growers of medical marijuana untouched. “The message from the Bush Justice Department was ‘watch out  we have the authority to go after everybody,’ ” he said.

On other matters, in his first wide-ranging conversation with reporters as attorney general, Mr. Holder said the Justice Department was still reviewing the case files of detainees held at Guantánamo Bay, Cuba, to determine whether they could be released or would be tried in a civilian criminal court or some other legal forum. He said it was possible that some detainees like the Uighurs held in Cuba could be released into the United States.