There is disclosed micro-processor controlled bio-fluid assay apparatus wherein microtitre plates are on carriers having machine readable labels and wherein the samples of bio-fluid and reagent dispensers also preferably carry machine readable labels whereby the micro-processors which controls movement of the plates through the apparatus can verify correct operation thereof. Movement of the plates is effected by a plate carrier transfer mechanism which has the ability to move the plate carriers in any order and in either direction along each of the x,y and z axes.