Release of immunoglobulin-binding factors was induced from cultured human lymphoblastoid cells in a serum-free environment by incubation at 37 degrees. RPMI-8866 cells expressing Fc receptors for IgE (Fc epsilon R) and Daudi cells expressing Fc receptors for IgG (Fc gamma R) released IgE- and IgG-binding factors, respectively. The factors induced were shown to have: (i) specificity of rosette inhibition; (ii) specificity of Ig-binding; and (iii) a glycoprotein nature. The IgE-binding factor remained stable, whereas the IgG-binding factor showed a time-dependent degradation and a concurrent change in specificity from IgG- to IgE-binding. The occurrence of this change was paralleled by a disappearance of a 125,000 mol. wt. protein on autoradiograms of SDS-PAGE of the IgG-binding factor preparation. These results raise the possibility that the released IgG-binding factor is found in a polymerized form which may dissociate into subunits in aqueous solution, and that common subunits may exist with regard to IgE- and IgG-binding factors, some of which may either lose their activity, or bind to both IgE and IgG with different affinities.