A method for determining boundary points on electrophoretic patterns whereby boundary points can be easily and accurately added onto or erased from fractionated patterns or analogue patterns by utilizing a densitometer for obtaining fractionated patterns of serums formed by an electrophoresis, a computer for making necessary operations and data treatments, an input terminal device for putting boundary point position informations or the like into the computer and an output terminal device which can record and digitally type analogue patterns with the output of the computer and means for working the method. Any input error by the input terminal device can be corrected.