This paper deals with studies on 3 specimens of serum that manifested the albumin autoagglutination phenomenon. Albumin autoagglutination is the result of a globulin present in certain human sera in conjunction with a substance, apparently a denatured protein, that is found in commercially processed concentrated preparations of albumin. Although the globulin property differs from an antibody, in that it can not be absorbed from the serum, the autoagglutinating activity can be absorbed if concentrated processed albumin is added to the serum before absorption. This behavior is suggestive of the formation of a denatured protein-globulin complex that is absorbed to the surface of the red blood cells during the agglutination reaction.