B-cell growth factor (B-cell growth factor I or B-cell-stimulating factor, provisional 1) is a differentiation factor for resting B cells and may not induce cell growth.

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Abstract
B-cell growth factor I [BCGF I or B-cell-stimulating factor, provisional 1 (BSFp1)] was defined as a T-cell-derived lymphokine that acts as a co-stimulator of polyclonal B-cell growth in [mouse] B cells cultured with anti-.mu., anti-.delta., or anti-Ig. Anti-Ig induces cell enlargement, entry into the G1 phase of the cell cycle and expression of receptors for BSFp1. BSFp1 then induces entry of the cells into S phase. By adding BSFp1 prior to anti-Ig, BSFp1 renders cells susceptible to anti-Ig-mediated entry of cells into G2/S phase. If cells are first treated with anti-Ig, washed, and then cultured with BSFp1, they do not enter S phase. BSFp1 acts on the resting B cells not as a growth factor but rather as a lymphokine that prepares cells for anti-Ig-mediated activation. BSFp1 induces increased expression of Ia antigens on resting B cells. BSFp1 may be a differentiation factor rather than a growth factor and that it acts on resting B cells.