Analysis of cellular polypeptides following separation by electrophoresis on polyacrylamide gels often takes advantage of radioactive isotopes that are introduced into the polypeptides during cell growth. Comparisons of polypeptides between differing organisms conveniently involve the use of two radioactive labels such as 3H and 14C. The gels on which the polypeptides have been separated are cut into 1 mm slices and the radioactivity associated with each slice is determined and expressed as 3H or 14C counts per minute (cpm). This paper describes a simple method of detecting differences in the polypeptides in the two differently labeled organisms. After the observed counts are suitably transformed to stabilize the between-slice variability, we compute the regression of the transformed 3H counts on the transformed 14C counts. For those slices corresponding to polypeptides that are the same in both organisms, the regression line should give a good fit to the observed data. Where the polypeptides of the organisms differ, the fit of the regression line should be poor. Hence, we examine the deviations from the regression line to assess the differences in polypeptides between the organisms.