Hiding a lookup field on a CRM 4.0 form

I'm trying to hide a look up field on a CRM 4.0 form. I've tried the 2 following lines, but neither works, I can still see the field on the form! It's definitely not being unhidden anywhere else.

crmForm.all.transactioncurrencyid.style.display= "none";

crmForm.all.transactioncurrencyid.style.visibility = "hidden";