Thank you for your help, it works perfectly!
But now I have another problem :-(
My ComboBox needs to fire a PostBack on index changed and I need to get some attributes from server-side. Before the use of WebService it was easy to do the job because of SelectedItem object.
Here is a part of my WebService:
How can I get the value of these additional attributes when the OnSelectedIndexChanged event is fired?
Thank you for your help