The central bank in Iran says defaced banknotes are to be made invalid, following an opposition campaign of anti-government slogans being written on the currency.
The Jam-e Jam newspaper quotes the bank as saying people who possess defaced notes should exchange them by 8 January.
Messages which support opposition leaders and denounce the government, have appeared on banknotes since a disputed election in June.
President Mahmoud Ahmadinejad was re-elected in the poll - mass street protests have since been held by the opposition who claim the election was rigged.
The slogans read: "Death to the dictator".
