Abstract: Apparatus for the in situ hybridization of slide-mounted cell samples contains modules for the three processes required: (a) reagent metering and spreading on a slide (1) which is effected by drawing a quantity of reagent (56-59) into a reservoir (31) and then spraying it on the slide (1) using air pressure. The rate of flow through the spray is ascertained by measurement and the dosage determined by timing the spraying; (b) slide washing; and (c) slide incubation. The slide washing and incubation are performed in the same dual-purpose housing (16). A high humidity level is maintained in the housing during incubation so that the samples do not need to be covered to prevent them drying out. A plurality of slides (1) are mounted in a carrier (11) to be washed and incubated at the same time.