Private Sub Workbook_Open()
Dim cellule As Range, der_col As Long, ws As Worksheet
Set ws = Worksheets("feuil1")
der_col = ws.Cells(6, Columns.Count).End(xlToLeft).Column
For Each cellule In ws.Range(Cells(5, 11), Cells(5, der_col))
If Left(cellule, 1) = "-" Then MsgBox "La cellule " & cellule.Address & " est négative !"
Next cellule
End Sub