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Highly purified T cell replacing factor (TRF or BIF) essentially free of IL-2 and Interferon activities is obtained in the invention. BIF is useful for treatment of immunoregulatory disorders and/or stimulation of production of immunoglobulins. A method for production of BIF and BGF is described. B cell lines for BIF assays are described as well as an assay for B cell receptor variants and a method for obtaining B cell variants from a variety of source materials.