The existence of a molecule responsible for the induction of Fc receptor (FcR) on bone marrow cells (FcR inducer, FcRI) is demonstrated in conditioned media from the macrophage-like cell line WR19M.1 activated by bacterial lipopolysaccharides. The molecular weight obtained from molecular sieving chromatography in gel and density gradient sedimentation is found to be 18,500 daltons and 16,000 daltons, respectively, with an isoelectric pH of 7.4. The factor is found to be thermolabile and trypsin sensitive. The macrophage and granulocyte inducer (MGI), also known as colony-stimulating factor (CSF) or colony-stimulating activity (CSA), is identified from the same source and found to have a molecular weight and an isoelectric pH different from FcRI. The fractions that contained the MGI did not induce FcR on bone marrow cells, while the fractions rich in FcRI did not induce colony formation.

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