To identify the proteins that are responsible for the electrophoretic pattern on cellulose acetate, we propose a method based on the precipitation of various individual proteins at the beginning of the electrophoretic run. Any non antibody proteins in the monospecific antisera were removed with a preliminary electrophoretic fractionation of the antiserum. Using this method, we could remove a band or a zone from the pattern, leaving the rest of the pattern unaltered. This permitted us not only to locate a protein in the pattern, but also to better interpret the electrophoretogram. The method requires no special apparatus and it is fast and cheap.