An automatic assaying system having a multiplex pipette cartridge section in which pipette cartridges are docked. At least one pipette cartridge has a different pipetting characteristic from another corresponding pipette cartridge, the different pipetting characteristic being selectable from a number of different pipetting characteristics. A multiplexing work item holder has an array of work item holder docks that dock corresponding work item holders selectable from pipette trays and pipette tip set racks that define the selectable different pipetting characteristic. One or more of the work item holder docks are indexed to selectably multiplex both the different interchangeable pipette trays and the at least one pipette set rack. A carrier moves the pipette cartridge or a work item holder carriage and a controller selects the at least one pipette tip set rack corresponding to a work item holder dock so as to effect automatic selection of the different pipetting characteristic.