IgM class antigen receptors present as an integral membrane protein on human B lymphocytes are isolated by affinity chromatography following surface-radioiodination, and lysis of cells by non-ionic detergent treatment. They are further purified by gel filtration and isoelectric focusing for detailed structural studies to explore any difference between membrane-bound Ig and their counterparts in secretions as well as to examine the mechanism of attachment of surface Ig to the lymphocyte membrane. Similar studies are extended to the other class (IgD) of antigen-receptors. Structural studies of IgD class and IgM class antigen receptors are intended to clarify whether or not the idiotypic specificity shared by them has a biochemical basis. Furthermore, the possible roles of two different classes of Ig in immune response in general will be investigated by specific antisera elicited against these surface molecules.