Fas was initially described as a molecule expressed on the surface of certain cell lines that could mediate programmed cell death (apoptosis) subsequent to ligation by specific mAb. To determine whether mAb to other epitopes on the Fas molecule might mediate other functions, we generated a panel of mAb to the extracellular portion of human Fas. Significant lysis of Fas-expressing target cells was only observed when the new mAb were first bound to a solid-phase support and not when the mAb were added in solution. However, several of these mAb inhibited the killing of target cells induced by the prototypic Fas-specific mAb, CH-11. Those mAb that inhibited apoptosis of target cells mediated by the CH-11 mAb also blocked lysis of target cells mediated by cells expressing Fas ligand. Finally, some of the Fas-specific mAb were found to co-stimulate proliferation of peripheral blood T cells in the presence of immobilized CD3 mAb. Thus, the data indicate the existence of a complex set of interactions mediated by Fas in both normal and transformed lymphoid cells that may have important implications regarding the role(s) of this molecule in regulation of immune responses.