TVIg are increasingly used Tor the treatment of autoimmune diseases. In the present study, we show thai IVIg contain antibodies directed againsl CD5, a cell surface molecule of T cells which is also a marker of the autoantibody‐producing CD20⁺(‘B‐r⁾ subset of B lymphocyies, Antibodies to the CD5 molecule were demonstrated in IVIg by the ability of therapeutie preparations of IVIg to inhibit the binding of labelled CD5 MoAh to ihe CD5‐expressing human Tcell lineH9. Prcincubation of H9 cells with IVIg or with F(ab')₂ fragments prepared from IVIg resulted in dose‐dependent inhibition nf the binding of CDS antibody. The presence in IVIg of antibodies to the CD5 molecule was fiirther confirmed by the binding of IVIg to mouse L cells that expressed human CD5 molecules following a stable transfeetion with CD5 cDNA. Human CD5 antibodies in IVIg provide therapeutic immunoglobulin preparations with the potential of modulating Tcell functions through CD5. and of regulating the expression of B cell subsets expressing CD5. This may have impliealions for the treatment of autoimmune diseases.