On partition in aqueous polymer two-phase systems containing dextran, poly(ethyleneglycol) (PEG), and PEG substituted with charged or hydrophobic groups "inagglutinable" ox erythrocytes showed more negative surface charge and less hydrophobicity than "agglutinable" ox erythrocytes. Tryspin treatment of the erythrocytes increased the agglutinability, reduced the negative charge, and seemed to increase the liability to hydrophobic interaction. Haemagglutination of highly negatively charged ox erythrocytes is almost impossible to accomplish by antibody against the ox erythrocytes alone or against IgG antigen linked to the erythrocytes in passive haemagglutination. Likewise, reverse passive (antiglobulin) haemagglutination cannot be accomplished with these cells when immunoglobulin antigens of moderate molecular size (less than or equal to 900,000) are used. However, these cells may be agglutinated when a non-charged carrier such as a bacterium coated with sensitizing antibody (immunoglobulin) is used to bridge the antiglobulin-coupled erythrocytes in a mixed latticer aggllutinate. Such a bridging can also be accomplished by heat aggregation of the immunoglobulin antigen.