using both auto post back and validators

[BabyJ]

have a pulldown with both AutoPostback set to true and a validator.When I select an "invalid" value from the pulldown, the validator messageshows briefly, then a postback occurs and the errormessage is gone!this is my code:\[code\]<asp:TextBox ID="SerialFrom_txt" runat="server" BackColor="White" BorderColor="#188F3E" BorderStyle="Solid" BorderWidth="3px" Width="200px" OnTextChanged="SerialFrom_txt_TextChanged" AutoPostBack="true"></asp:TextBox> <asp:RegularExpressionValidator ID="SerialFrom_rv" runat="server" ValidationExpression="^[a-zA-Z]{2}[0-9]{7}$" Text="error" ControlToValidate="SerialFrom_txt" />\[/code\]and my behind code for post back is:\[code\]protected void SerialFrom_txt_TextChanged(object sender, EventArgs e) { if (string.Compare(RoleFrom_lbl.Text, SerialFrom_txt.Text) > 0) { Error_lbl.Text = "Serial From must be greater than " + RoleFrom_lbl.Text + "!"; SerialFrom_txt.BackColor = System.Drawing.Color.FromName("red"); SAVE_btn.Enabled = false; } else { Error_lbl.Text = ""; SerialFrom_txt.BackColor = System.Drawing.Color.FromName("white"); SAVE_btn.Enabled = true; } }\[/code\]so please tell me what should I do?