Many transactions are carried out in cash in Iran
Iran's central bank has said defaced banknotes are to be made invalid, after the appearance of opposition slogans on money, local media report.
They quote bank official Ebrahim Darvishi as saying that people possessing defaced banknotes should exchange them by 8 January.
It was difficult to distinguish genuine defaced notes from fakes, he added.
Messages in support of the opposition have been appearing on banknotes since June's disputed presidential election.
Hardline President Mahmoud Ahmadinejad was re-elected in the poll - a move that triggered mass street protests by the opposition who claimed the election was rigged.
'Death to dictator'
"Banknotes on which there are writings or are stamped or have any additional signs will be invalid," Iran's Jam-e Jam newspaper quoted Mr Darvishi as saying on Thursday.
The bank official also called on the public and shop owners not accept such notes from 8 January.
Since June's election, slogans such as "Death to the dictator" have appeared on many Iranian banknotes.
Opposition supporters have also sprayed anti-government graffiti on walls in the capital Tehran and other cities.
Many transactions are carried out in cash in Iran, which is under international banking sanctions and where major credit cards cannot be used.