It has been difficult to detect IgG-rheumatoid factor (IgG-RF) by hemagglutination procedures. This difficulty was thought to be due to the size of IgG-RF being too small to break Z-potential on the red blood cell surface. Therefore, a new method for detection of IgG-RF in sera was devised using sheep red blood cells sensitized with rabbit gammaglobulin, and antihuman IgG antibody. Fundamental experiments were performed. The treatment of a few kinds of 19S-IgM hemagglutinins, i.e., heterophile antibody, anti-B antibody, cold agglutinin and IgMRF with 2-ME reduced hemagglutination activity. Furthermore, the addition of anti-IgM antibody restored that activity. These observations suggest that IgG-RF alone does not agglutinate sensitized sheep red blood cells, but that it might agglutinate them in the presence of antiIgG antibody.