Evidence of the existence in human plasma of an activity analogous to that of bovine conglutinin is presented. The human plasma component was characterized antigenically and functionally. Human plasma was shown to agglutinate complement-coated erythrocytes in the presence of Ca2+, and this conglutination was inhibited by EDTA. The molecule also binds to complement-reacted solid phase IgG and to zymosan in the presence of Ca2+. The binding to complement is not inhibited by N-acetyl-D-mannosamine, but is inhibited by N-acetyl-D-glucosamine, as previously shown for bovine conglutinin. Antibodies raised against bovine conglutinin cross-react with the human protein. The plasma concentration of the conglutinin-like protein showed a high inter-individual variation between apparently healthy donors. Unlike bovine conglutinin, the human conglutinin activity could not be demonstrated in serum but only in plasma. The activity was more stable in plasma containing metal-ion chelators like EDTA and citrate than in heparin or hirudin plasma.