String.Replace not working [Resolved]

[salvelinus]

In the code below, the .Replace function appears to run w/o error when I step through the code, but it doesn't replace the string. The original string is left intact. What's going wrong? Thanks.

Private Sub SetQuestionSql(ByVal cb As CheckBox)
        Select Case cb.Name
            Case "cbModule"
                If cb.Checked = True Then
                    If mblnChanged Then
                        mstrQuestionSQL &= " AND modcode = "
                    Else
                        mstrQuestionSQL &= " WHERE modcode =  "
                        mblnChanged = True
                    End If
                Else
                    If mstrQuestionSQL.IndexOf("AND modcode = ") <> -1 Then
                        mstrQuestionSQL.Replace("AND modcode = ", "")
                    Else
                        mstrQuestionSQL.Replace("WHERE modcode = ", "")
                    End If
                End If