Thank you for writing.
The RadAutoCompleteBox control internally uses the RadTextBoxControl, which does not support right to left yet. We do have a feature request for this. Make sure you cast your vote for it by clicking the like button: http://feedback.telerik.com/Project/154/Feedback/Details/111258
I hope that you find this information useful. Should you have any other questions, do not hesitate to contact us.
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