a computer controlled pipetting system with a hand held pipettor and a control assembly . the pipettor has a motor driven pump for aspirating and dispensing liquid through a pipette which is inserted to the pipettor . the control assembly has a keyboard for controlling operation of the pipetting system . a calibration pipette is inserted into the pipettor for establishing reference signals corresponding to a selected density and temperature of a liquid which is aspirated into the calibration pipette by the pump . the reference signals are stored in a memory and subsequently used for aspirating selected quantities of liquid into a laboratory pipette which is inserted into the pipettor in place of the calibration pipette .