Do you mean you want to be able to reference these cells in lookup formulas
contained in other cells?
If so, leaving these cells locked (Format | Cells | Protection), and then
Protecting the Sheet (from teh Tools Menu) should be enough.
You may want to make sure you have Select Locked and Select Unlocked Cells
Checked, if a list of Protection Options pops up (Excel 2002 and later).
tj