An automatic liquid transfer system includes a horizontally translatable table and a vertically translatable set of pipettes. The table accommodates a titer tray having a multiplicity of receptacles to be filled, or holding liquid samples to be diluted, and a rack housing plural rows of disposable tips. During each cycle in a serial dilution process, a fresh set of tips are picked up by the pipettes and used to transfer liquid in a sterile manner from a sample or diluent source to a row of wells in the titer tray, or from one row to a succeeding row of wells where it is mixed with diluent. Thereafter, the tips are discharged back into the rack to maintain sterile conditions during the process. A sensor is provided on the machine to detect whether all of the tips in each set are disengaged and another set successfully picked up.