A delivery apparatus for selectively delivering one or more liquid reagents into a reaction or test chamber (2), especially of an assay apparatus, the apparatus comprising: one or more respective storage chambers (5,6) for containing the one or more liquid reagents and arranged generally above the reaction or test chamber (2); and a plunger element (4) arranged and operable for insertion into the mouth of a selected storage chamber so as to displace a selected reagent from therewithin into the reaction or test chamber (2) generally therebelow by gravitational liquid overflow from the mouth of the chamber. The apparatus may conveniently be provided as a discrete delivery unit, with the storage chambers (5,6) pretilled with the selected reagents.