Hi Jennifer
I think there are several ways to do this.  Look at the Helper Table in the ABCD by making a choice you open up a list of options this may give you an idea.  Or just do in in query two fileds on the protocol and details filter the details based on protocol.
Let me know if this helps

Sorry Juan, you've lost me here..
I'm still very much a newbie. How can I share the DB with you so I can get a little more guidance?

Usually we wouldn't do sharing databases Jennifer, see Forum Posting Rules Rule 5. That would be something more for the DevNet.

See Cascading this should help you out.

You may also need to know Value From a Form depending on your setup if you're using a subform.

When I mentioned the helper table I was thinking of how Richard had two tables set up.  So let's say table 1 has your protocols.  table two has Protocols and material.  Set up a filter on table two from the selection of table one.  this should just display the material for that protocol or query and criteria using a combo box...in my mind, I am thinking of how to do this but having a difficult time explaining it.  let me know if this helps.  can always set up a chat and walk you through it if you like.

Thanks Adam and Juan. I wasn't aware of the posting rules and don't want to upset anyone :)
I am sorry for the long post, but I am trying to be specific on the issue.

I have looked at the Cascade and Value From a Form videos, and I got part of the way there, but I am still having issues.

I have already successfully created a query (Invoice Query1) producing the correct values from the forms using the value from a form as the criteria.
I am pulling MaterialID (the index), MaterialName (the value required) and MaterialNumber (to display only) from the Material table and ProtocolID from the Protocols table.

In it I have MaterialID (the linked index, the Material Name (which I want to store in the invoice details subform and display) and Material Number (which I just want to display in the dropdown)) from the Materials table and ProtocolID from the InvoiceDetails table, with criteria [Forms]![Invoices]![ProtocolID]. (they are linked fields between form and subform).

When I create the combo box in the subform and try to use the query get the values, it brings up a message saying:
"When you select a row in the combo box, you can store a value from that row in your database, or you can use the value later to perform and action. Choose a field that uniquely identifies the row. Which column in your combo box contains the value you want to store or use in your database?"
Should this be the Material ID (the index) or the MaterialName (the actual value to be shown) field?

I then get the usual message of "Microsoft Access can store the selected value from your combo box in your database, or remember the value so you can use it later to perform a task. When you select a value in your combo box, what do you want Microsoft Access to do?"
I thought this should be the Material field (pls correct me if I am wrong), but it also adds to fields to the list called ProtocolID_Materials and ProtocolID_Materials1 to the list. I have no idea what these are..

If I select Material ID in the first message box, then store in Materials, it stores the index only. This creates 2 problems. First, the user can't type in the material name to select the option, they must use the dropdown selector. I have tried setting the column width to 0 for the index column as I have for my other cascading combo boxes, but then it blanks out the dropdown selections altogether!
Secondly, it is ugly for the user. As it is a datasheet view, I don't have the option to overlay a text field to make it pretty.. any thoughts on this would be appreciated.

If I select MaterialName in the first message box, then store in Materials, I get the "The value you entered isn't valid for this field..." and it won't let me select anything.

Any help you could offer here would be great. Thanks Jenny.

Further from this, I managed to produce the combo box with Material ID as the bound column and made column width 0.
It worked fine, until I shut the database down. When I reopened it, the dropdown now just shows 1 blank row.
How do I get it look for the query each time the DB fires up?