A convenient method was established for the M(r) estimation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of large polypeptides in the range from 250,000 to 600,000. Such polypeptides had previously been shown to migrate improperly in various gel electrophoresis systems. In a conventional Laemmli system with 3% polyacrylamide, approximately linear log M(r) vs Rf plots could be obtained by varying the gel thickness which ranged from 0.4 to 1.5 mm. In the additional presence of an electrolyte, such as NaCl, running times were shorter and for evaluation linear plots could be constructed from runs in very thin gels with less than 0.5 mm thickness. Handling of gels was greatly facilitated by fixing them to one of the gel chamber glass plates via bind-silane treatment prior to casting.