The Irish Republican Army (IRA) has decommissioned weapons in the past, but even so is still thought to have lots left.
The Northern Ireland secretary Paul Murphy said the move from the UFF and UDA was a "step in the right direction".
The two groups who made the announcement are Unionist, which means they want Northern Ireland to stay part of the United Kingdom.
The Republicans, also called Nationalists, want Northern Ireland to join Ireland to become one united country.
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