Four sera with the albumin autoagglutinating property were studied. This phenomenon is characterized by pan- and autoagglutination of erythrocytes only in an albumin medium. The agglutinin is a gamma globulin which reacts specifically with caprylate-treated albumin. Sodium caprylate and acetyl tryptophan are added as stabilizers in the commercial preparation of therapeutic human albumin and most blood bank reagent albumins to prevent denaturation when the albumin is heated. The authors suggest that agglutination results from adsorption of a soluble albumin–globulin complex onto the erythrocyte membrane. As native albumin is not reactive with these agglutinins, patients with the abnormality should encounter no hazard with transfusions of whole blood or plasma. Therapeutic human albumin and plasma products containing stabilized albumin are probably contraindicated in these patients. The occurrence of an immunoglobulin reactive with commercially altered albumin emphasizes the importance of knowledge concerning additives to therapeutic and reagent biologic products.