CD4 and CD8 exist in membrane-bound and soluble forms. These antigens are the physiological ligands for molecules of HLA classes I and II, which we have found in soluble form in commercial immunoglobulin preparations. We measured concentrations of soluble CD4 and CD8, as well as soluble HLA-ABC and HLA-RQP in sixteen immunoglobulin preparations by enzyme immunosorbent assays. Only two preparations contained detectable CD8, and nine had soluble CD4 in varying amounts. There was great variation in concentrations of soluble HLA molecules. It is possible that the immunomodulating effects of immunoglobulin therapy in disorders with immune aetiology are due, at least partly, to these contaminating molecules.