The albumin agglutination phenomenon is due to antibodies which cause agglutination of all human red cells when these cells are suspended in an albumin medium. Two sera with this property were studied. We suggest that the reaction is due to non-specific adsorption of antigen-antibody complex onto red cells. The antibody is a gamma globulin directed at albumin which has been altered by the addition of acetyl tryptophanate or caprylate. These chemicals are added as stabilizers in the manufacture of albumin to prevent dénaturation when the albumin is heated. Since unaltered (native) albumin does not react with these antibodies, blood or plasma transfusion to patients with this serologic abnormality should present no unusual hazard. A non-reactive albumin should be used in crossmatching blood for such patients. Therapeutic human serum albumin and plasma products containing stabilized albumin are probably contraindicated in these patients.