Abstract: A novel established cell line, KML.sub.1-7, obtained by collecting lymphoid cells from an animal with autoimmune disease, culturing them in a culture medium containing the supernatant of a culture in which lymphoid cells of animal origin have been grown in the presence of a mitogen, and continuing the culture for a period of 8 months until the cells are able to grow in the absence of the aforesaid supernatant. This established cell line has the following properties.(1) It produces a factor, B-Cell Differentiation Factor (BCDF), participating in the differentiation of antibody-producing cells.(2) It does not produce B cell growth factor.(3) It consists of immortalized null cells.