A syringe pump has a drive mechanism for moving the plunger of a syringe along a barrel. Ears at the rear of the barrel engage against a wall on the pump and an L shape clamp arm is urged against the ears by a spring. The clamp arm is mounted about an axis at right-angles to the syringe and is articulated at a point along its length. A lever having a flag at one end is pivoted about a horizontal axis by movement of the arm and moves relative to an optical sensor so that the output of the sensor indicates whether the syringe is correctly loaded in the pump. The clamp arm is rotated away from the barrel by the drive mechanism during loading of the syringe.