Sub test()
Application.ScreenUpdating = False
With Range(Cells(1, 1), Cells(16, 1))
'si c'est toute la colonne A mettre cette ligne ci dessous
'With Range(Cells(1, 1), Cells(65536, 1).End(xlUp))
'et supprimer le If
If ActiveCell.Row = 1 Or ActiveCell.Row = 6 Or ActiveCell.Row = 11 Then
With Selection.Validation
.Delete
.Add Type:=xlValidateTextLength, AlertStyle:=xlValidAlertStop, _
Operator:=xlEqual, Formula1:="28"
.IgnoreBlank = True
.InCellDropdown = True
.InputTitle = ""
.ErrorTitle = ""
.InputMessage = ""
.ErrorMessage = ""
.ShowInput = True
.ShowError = True
End With
End If
End With