We have prepared an mAb specific for a human cell surface component (termed anti-Fas mAb). Anti-Fas shows cell-killing activity that is indistinguishable from the cytolytic activity of TNF. Fas antigen was characterized by western blotting, indicating that Fas antigen is a cell surface protein with a molecular weight of 200,000, which is different from the molecular weight of TNF-R. Fas antigen, however, is co-downregulated with the TNF-R when cells sensitive to the cytolytic activity of TNF are incubated with either TNF or anti-Fas. In contrast, Fas antigen on cells insensitive to TNF is not co-downregulated with the TNF-R. We suggest that the cell-killing activity of TNF is mediated by Fas antigen associated with the TNF-R.

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