To: xander, I didn't think rows in a combo box could be disabled. My solution therefore takes a different turn: when an item in combo box #1 (a category-type field from the same table as the detailed data in combo box #2) is clicked in combo list box #1, the rowsource for box #2 is changed, to display only items in the selected category.
Me!cboBOX2Data.RowSource = "SELECT DISTINCT [MyTable].[DescriptionField] , [MyTable].[CodeField] FROM [MyTable] WHERE [MyTable]![CategoryField] = [Forms]![MyForm]![cboBOX1Categories] ORDER BY [MyTable].[DescriptionField];"