A computer program is described that will determine the molecular weight of DNA, RNA or protein molecules separated according to size by gel electrophoresis. It uses the sizes and migration distances of known molecules in a reference lane to compute a second or third order equation whose curve best fits the data points. It then computes the sizes of all molecules from this equation. Migration distances are measured and entered using an analog tablet. The program is written in Apple Pascal and designed to run on an Apple II Plus computer.