We are indebted to Dr. Eloise R. Giblett, Associate Director of the King County Central

A rapid system for the quantitative fractionation of serum proteins by electrophoresis on cellulose-acetate membranes was evaluated and found to be quite precise. The reproducibility (coefficient of variation) of the routine fractionation of a control serum carried out 40 times during a 15-week period was 2.4 % for albumin and 14.2, 6.0, 6.1, and 5.2%, respectively, for the ar-, a2-, /3-, and-y-globulin fractions. Normal values are given for serum protein fractions (specimens from nonprofes-sional blood donors) obtained by cellulose acetate electrophoresis. A.LTHOUGH PAPER ELECTRO1’HORESIS is a widely used technic for the fractionation of serum proteins, cellulose-acetate membranes are su-perior to paper as a Su))Ortiflg medium (1-7). Some advantages of cellulose acetate over paper are as follows: 1. The regularity of pore size results in sharp resolution of fractions in a relatively short time. 2. There is little adsorption of protein to the membrane, thereby increasing tile resolution and eliminating the ‘‘trailing’ ’ of albumin. 3. The quantitation of protein-bound (lye by densitometric scanning is more accurate because the protein is more evenly distributed in the fine membrane structure than in irregular paper fibers. 4. The membranes are rendered transparent easily, thereby increas-ing the accuracy of the scanning process. 5. Wet cellulose acetate membranes have greater tensile strength. Despite these advantages, cellulose acetate has not readily replaced paper as a supporting medium iii clinical chemistry laboratories because too much time has to be consumed in the individual handling of tile cellulose-acetate strips during the dyeing and clearing process. Als