1) use a query to fill your combo
2) filter the query, e.g. hFilter(myQuery, "myItem = " + myOtherComboValue), or myOtherComboValue..DisplayedValue
3) re-display combo (ListDisplay for file combos, FileToMemoryList for memory combos)
As the query is in memory, filtering is very quick even with relatively large lists.
Regards
mat

You didn't say that your combo was part of a table. If your main group is already selected when you enter a new stock, all you should need to do is re-display your table combo. I'm not very familiar with combos in tables and imagine that you have to activate the filter when entering the combo and deactivate when leaving it. Since the sub group table combo and list potentially contain different values, you need to use a different data source (e.g. two identical queries). When refreshing display, you might have to add the table row number, using TableSelect, in square brackets, something like ListDisplay(myTable.myCombo[TableSelect(myTable])). When refreshing combo display, you probably also have to re-assign the original field value.