Sera from 16 of the 25 patients with PNH were found to exhibit anti-γC globulin activity as demonstrated by their ability to agglutinate human red cells sensitized with certain incomplete Rh antibodies. Of the 16 active sera, 9 agglutinated human FII-coated tanned sheep cells and 5 FII-coated latex particles in significant titer. Only one serum exhibited anti—Gm specificity. In 3 cases serum anti-γG globulin activity was found prior to transfusion of the patient, suggesting that isoimmunization is not in all cases responsible for this phenomenon. The chromatographic behavior of the serologically active substances and their inactivation by treatment with 2 ME suggest that they are γM globulins related to the rheumatoid factors. The possible role of these antiglobulin factors in the lysis of the PNH erythrocyte is discussed.