Bill Renaud wrote:
I forgot to add that this code works only as long as the worksheet
has NOT been protected yet. If the worksheet has been protected, then
an error will be raised.

I rewrote the routine slightly as:

'----------------------------------------------------------------------
Public Sub FormatMarksRange()
Dim wsMarks As Worksheet
Dim blnProtected As Boolean
Dim rngMarks As Range

Set rngMarks = Range("Marks")
Set wsMarks = rngMarks.Parent
blnProtected = wsMarks.ProtectContents

If blnProtected Then wsMarks.Unprotect

With rngMarks
.FormatConditions.Delete

.FormatConditions.Add Type:=xlExpression, _
Formula1:="=CELL(""protect"",RC)"
.FormatConditions(1).Interior.ColorIndex = 24
End With

If blnProtected Then wsMarks.Protect
End Sub

Excellent. Thank you, Bill, for your clear explanation of RC and your
suggested enhancements to my routine. This is clearly the way to go rather
than an iterative solution.