The hollow cylindrical syringe body has a channel of uniform diameter. A fluid port is situated at one end. The other end defines a funnel-like opening with a diameter greater than that of the channel. A piston is moved along the channel by the end of a push rod which is received through the channel opening. A ball on the push rod lodges in a socket in the piston as the rod pushes the piston toward the port. The socket is formed of resilient fingers which are held by the channel wall in position to retain the ball in the socket, such that the piston can be moved along the channel to load the syringe. When the socket aligns with the funnel-like opening of the channel, the socket fingers spread apart to release the ball from the socket, permitting the push rod to be removed from the loaded syringe.