Wash you, make you clean; put away the evil of your doings from before mine eyes; cease to do evil; (KJV)
The repentance motif is continued in the next few verses, including the great promise of forgiveness in verse 18:
Come now, and let us reason together, saith the Lord: though your sins be as scarlet, they shall be as white as snow; though they be red like crimson, they shall be as wool.
Is there a specific rite or ordinance to which Isaiah refers? It is curious to me that these passages come just after the Lord says:
To what purpose is the multitude of your sacrifices unto me? (verse 11)
Bring no more vain oblations (verse 13)
What kind of washing does Isaiah call for?