Human monocytes, when appropriately stimulated in vitro, release into the culture medium a factor (BAF) that stimulates the IgM response of T-depleted murine splenocytes to heterologous erythrocytes. The behavior of this factor on gel filtration, isoelectric focusing, ion exchange chromatography, and isopycnic centrifugation was studied. BAF appears to be a molecule of 15,000 daltons, pI 6.5, 1.33 g/ml with low solubility at low ionic strength. It is stable to acid, mild heating, and long-term storage. Activity is lost in alkali or by boiling. Papain may reduce BAF activity slightly, whereas trypsin and chymotrypsin have no significant effect. These properties are similar to those of other monokines reported to have a similar m.w.

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