Abstract: An adaptor for retrofit attachment to a syringe has a slide slideably mounted on the barrel of the syringe, a housing for releasably engaging a flange formed on the trailing end of the syringe plunger, an elongate connector for interconnecting the slide and the plunger housing, and a guide that releasably engages a flange formed at the trailing end of the barrel. Movement of the slide effects simultaneous and corresponding movement of the plunger. This allows a syringe operator to perform aspiration and injection in sequence with one-handed operation of the syringe. The slide is adjacent the leading end of the plunger so that the operator can see the leading end of the plunger. The user's fingers support the syringe from below. The flange housing permanently engages the flange in a second embodiment. In a third embodiment, the elongate connector and plunger are formed integrally at their respective trailing ends.