What are valid us bills?

The U.S. has never withdrawn any bills issued as either Federal Reserve Notes, Silver Certificates, or U.S. Notes. Thus technically any bills in any of those series are still valid in retail commerce. But it's not that simple ....

From a practical standpoint, bills that would be commonly accepted today include only Federal Reserve Notes in denominations $1, 2, 5, 10, 20, 50 and 100.

U.S. Notes and Silver Certificates are often worth more than their face value to collectors (sometimes only a few cents, sometimes a lot) , so it would be foolish to spend them today.

High-denomination bills ($500 to $10,000) haven't been printed since 1945, and in 1969 a presidential order declared that banks didn't have to distribute them because they were being used by organized crime to hide large transactions. Between their higher collector value and the suspicions that their use would raise, high-denomination bills have been effectively (although not legally) rendered invalid.