A new method for molecular weight determination using polyacrylamidegel electrophoresis in a linear gel-concentration-gradient (3-20 degrees ) is described. Plotting the log of molecular weights of several standard proteins against distance of migration or against 3 log of rate of n-dgration reveal linear relationships in ranges of 50.000 - 200.000 or 100.000 - 400.000 Daltons respectively. On this basis, a simple method for molecular weight determination of proteins (accuracy +/- 5 degrees ) has been devised. The method can also be applied for an individual protein in protein mixtures using specific staining procedures.