A valve especially suitable as a relatively cheaply manufactured means for controlling liquids in laboratory equipment, comprises a body having a valve seat with a flow passage passing through the seat, and a spindle located within the body; the spindle comprising a mandrel having an annular groove, a resilient annulus located in the annular groove, and an inert sheath fitted closely over the surface of an end portion of the mandrel and overlying the resilient annulus, the sheath having a closed end shaped to engage the seat for occlusion thereof and having overlying the resilient annulus an annular sealing portion standing proud of the surface of the rest of the sheath, the annular sealing portion being biased against the surrounding body to form a sliding gland seal. The sheath may be preformed or moulded in situ, and protects the mandrel and resilient annulus from attack by fluids flowing through the valve.