A human B cell line （3D5） that responds specifically to B cell growth factor （BCGF） has been developed by a sequence of Staphylococcus aureus Cowen I activation

EB virus im- mortalization

and cloning.Proliferative response to PHA-stimulated T cell supernatant （PHA-T-Sup） and nonresponsiveness to rIL-2 stimulation were factors used to screen positive cells.Phenotype analysis with a flow cytometer indicated that:1) 3D5 is a B cell line: 100% of the cells were positive for B1 marker and 59% were positive for sIg

while T3 and Mo 1 were negative:2) 3D5 is an activated B cell line:both Tac and 4F2 markers of activated （but not of resting） B cells were 100% positive:3) 3D5 expresses high molecular weight BCGF （HMW-BCGF） receptor-associated epitope BA5.3D5 cells proliferated in response to cpBCGF stimulation in a dose-dependent manner.HMW-BCGF also induced 3D5 cells to proliferate.Interestingly.no proliferation could be detected in the presence of rIL-2

rIL-4

or rIFN-r.The data show that 3D5 cells are specifically BCGF-responsive B cells.Using 3D5 cells as target

BCGF activity was detected in crude BCGF preparation sedimented by 85% （NH

4

）

2

SO

4

and chromatographed in a DEAE-Sephadex A-25 column from PHA-T-Sup.T24 cell supernatant with B cell differentiation factor （BCDF） activity could not induce 3D5 cells to differentiate into immunoglobulin-secreting cells.