The error in quantitative gel electrophoresis/Western blotting was investigated considering the purity testing of erythropoietin. The overall error was over 35% relative standard deviation. However, an analysis of variance elucidated that the interoperator variability was the dominant error source, which already explained almost 80% of the total variance. Careful compilation and investigation of the possible error sources strongly indicates that the immunoreaction after blotting and the subsequent color reaction are the major error sources in this case.