A substance in the serum of the marine gastropod, Aplysia californica, capable of agglutinating marine bacteria and vertebrate red blood cells was subjected to physicochemical analysis in order to ascertain its possible nature. Our studies indicate that agglutinating activity is due to a heterogeneous group of high molecular weight molecules with two activity peaks exhibiting sedimentation coefficients centering ∼18.5 S and ∼31.0 S. This material has a protein component associated with the active site since it is sensitive to heat, pH extremes, and extraction with 2-mercaptoethanol, phenol, chloroform, and trichloracetic acid. Its physicochemical characteristics are different from other known invertebrate agglutinating substances and from classical vertebrate antibody. © 1971.