Gel filtration, the chromatographic separation of proteins on columns of gel particles of various nature, has empirically been found to constitute a useful tool for molecular size determination (l-6), as indicated by a variety of applications. Few attempts have been made to utilize the interaction of proteins and gel matrix during zone electrophoresis for the same purpose. Agar gel in the routinely used concentrations does hardly interfere with the electrophoretic migration of proteins, unless their size is very large (7). Higher percentages of agar show molecular sieve effects, but are not practical for electrophoresis. The high resolving power of starch gel electrophoresis is obtained by a combination of electrophoretic effects and molecular filtration, and Smithies (8) has shown that the mobility of proteins in this medium is related to gel concentration versus molecular size, allowing an estimation of the latter.
Polyacrylamide gel (9) has several advantages over agar and starch gel as a supporting medium. The absence of absorption and endosmosis combined with the optical clarity of the gel facilitate the accurate measurement of migration rates, and its chemical neutrality makes it very suitable for specific staining reactions. The technique is relatively simple and allows great variations in experimental conditions. Thus the concentration of the gel and consequently the size of the pores, which has an inverse relation to the square root of the concentration (lo), can be varied over a greater range than that of any other supporting gel. Ornstein (11) and Tombs (12) have suggested ways to use this for a determination of diffusion constants and molecular sizes, respectively. The work to be described here shows that the combination of properties of polyacrylamide gel permits a simple estimation of the molecular weights of proteins under certain experimental conditions, even though theoretical objections can be made against the followed procedure. The