Fas-mediated apoptosis plays an important role in the immune system, including the elimination of autoreactive lymphoid cells. The Fas-mediated signaling pathway is classified into two types, type I and type II, in human lymphoid cell lines. We investigated whether a humanized anti-human Fas mAb, R-125224, has cell selectivity in induction of apoptosis. R-125224 induced apoptosis in H9 cells, SKW6.4 cells and activated human lymphocytes when cross-linked with anti-human IgG. On the other hand, R-125224 did not induce apoptosis in HPB-ALL cells, Jurkat cells or human hepatocytes. By analysis of death-inducing signaling complex formation, it was demonstrated that R-125224 induced apoptosis selectively in type I cells but not in type II cells. Type I cells also expressed more Fas and had more Fas-clustering activity than type II cells. Moreover, co-localization of these clusters and GM1, which is an sphingoglycolipid associated with lipid rafts, was detected. It was also shown that R-125224 treatment could reduce the number of activated human CD3+Fas+ cells in a SCID mouse model in vivo. Thus, we demonstrated that R-125224 induces apoptosis specifically in type I cells in vitro and in vivo.