A needle spacer is disclosed for use with a biopsy needle assembly of the type which is coupled to a driving unit to effect movement of the needle assembly. The needle spacer maintains the biopsy needle assembly in a predetermined operational relationship until after the needle assembly is coupled to the driving unit. In the preferred embodiment, the needle spacer prevents the lid of the driving unit from being closed while the needle spacer is still in place on the needle assembly, thereby eliminating the possibility that the physician will forget to remove the spacer prior to firing the instrument. In a first alternate embodiment, the needle spacer is configured to interfere with movement of the needles within the driving unit while the spacer is still in place so that if the physician attempts to fire the instrument with the needle spacer still in place, the needles will not advance. In a second alternate embodiment, the needle spacer includes a readily visible flag which, even with the lid of the driving unit closed, will alert the physician that the needle spacer is still in place, thus reducing the possibility that the physician will forget to remove the spacer.