The US government has said it will not sue to stop the states of Colorado and Washington from legalising recreational use of marijuana.
The justice department said it would focus on preventing underage access to the drug and keeping drug cash from criminals, but declined legal action.
Voters in Colorado and Washington state legalised possession and use of small amounts of marijuana in November.
Federal law forbids marijuana use and possession.
In addition to the two states that outright legalised the drug, 20 states and Washington DC allow medical marijuana use, a stricter standard.
"For states such as Colorado and Washington... the department expects these states to establish strict regulatory schemes," the justice department said in a statement.
The department said it was "deferring its right to challenge legalization laws at this time" but said it could challenge the eventual regulatory schemes in the states.
Dan Riffle of the Marijuana Policy Project, a pro-legalisation group, called the announcement "a clear signal that states are free to determine their own policies".
The executive director of a group called the Drug Free America Foundation said Attorney General Eric Holder, an appointee of President Barack Obama who oversees the justice department, had "surrendered".
"He is essentially setting up a tsunami that will no doubt result in far too many of America's young people being subjected to chemical slavery," said Calvina Fay.