Hi,
Try this:
<iframe height="450px" width="450px" scrolling="auto" runat="server" id="iFrame1" />
protected void Page_Load(object sender, EventArgs e)
{
string Url = "TXT/business applications_en.htm";
System.Web.UI.AttributeCollection aCol = iFrame1.Attributes;
aCol.Add("src", "Test\test.aspx");
}
OR
Try this: add a hidden input field (with runat="server" attribute) and change it's value to your IFrame's src in the JavaScript. That way, you'll be able to read the field's value server-side.
--Amit