The expression and functions of Fas antigen, a major regulator of apoptosis, in T‐cell selection have been intensively investigated, but little is known about its expression in immature B cells which are also selected in the bone marrow, and plasma cells which are at the terminal stage of B‐cell differentiation and are designed to die. We examined bone marrow cells and found Fas antigen on these cells at low levels. Next, we analysed Fas expression and susceptibility to anti‐Fas antibody‐mediated apoptosis on B‐cell lines representing various stages of differentiation. We also examined the expression of Bcl‐2 and Bax on these lines, which were intimately related to apoptosis. Fas antigen was almost negative on pre‐pre‐B cell lines and was detected on pre‐B‐cell lines at low levels. All plasma cell lines expressed Fas at a low to moderate level. Some cell lines with peripheral B‐cell phenotype expressed Fas antigen. Except for the Burkitt cell lines and one plasma cell line, susceptibility to Fas‐mediated apoptosis depended on Fas expression. Bcl‐2 protein was detected on all but one cell line and Bax was detected on 15/23 lines, but neither were related to cellular differen
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