Abstract: A single-syringe device capable of inserting its contents, such as a blood filter, into the body of a patient in either of two opposite directions exclusively chosen by an operator in a fail-safe manner. First and second plungers slide in opposite directions within the syringe, which is connected to a catheter or similar, to transfer the contents into the patient's body. The device is constructed so that only one plunger can be employed at a time. Each plunger and the end of the syringe it cooperates with is coded, by color or otherwise. Such a construction and coding insure fail-safe operation when the syringe's contents must be inserted in only one direction. For example, a blood filter that traps emboli must have the same orientation whether it be inserted in one direction from the patient's jugular vein or in the opposite direction from the patient's femoral vein.