Surface active antibodies were formed by covalent attachment of hydrophobic groups to IgG molecules. The modification was carried out at various degrees of attachment and various chain lengths. These modified antibodies spontaneously formed new colloidal micelle-like particles, immunoclusters, which were viewed with a transmission electron microscope and evaluated by light scattering. We found that the size of the immunoclusters increased while either degree of attachment or chain-length were increased, and that the biological activity decreased with the increase of degree of modification.