Officials won't say where or when they found the stuff and it will be a few days until the results come through.
But some reports say 14 big unlabelled containers were found near Bayji, about 210 kms (130 miles) northwest of Baghdad.
They were discovered near missiles and gas masks, says the US.
The US is also testing what it says could be a mobile weapons lab.
The first tests show there could be traces of nerve and blistering agents or chemicals.
The problem is, many pesticides used on crops contain these agents too. So the US will have to wait until more thorough tests are done.
Suspicious materials which the US has found before have proved to be false alarms.