We have examined the activity of a new member of the tumor necrosis factor (TNF) family identified through Expressed Sequence Tag database searching using TNFalpha protein as the search query. We have termed this protein TNF-related death ligand-la (TRDL-1alpha). Traditional cDNA library screening identified two additional splice variants designated TRDL-1beta and TRDL-1gamma that differed from TRDL-1alpha by the deletion of two small regions within the protein coding region. TRDL-1alpha is identical in sequence to the recently described molecule, APRIL, that may induce cell proliferation. We found, however, that purified, FLAG-tagged TRDL-1alpha caused Jurkat cell death with kinetics that paralleled FasL. In vitro binding experiments demonstrated that TRDL-1alpha coprecipitated Fas and HVEM and suggested TRDL-1alpha as an alternate ligand for these receptors. TRDL-1 localized to chromosome 17p13.3 and its expression was widespread in normal tissues. Examination of 48 tumor samples revealed high levels of TRDL-1 expression in several tumors, including those from the gastrointestinal tract. Expression of TRDL-1 in COS-1 cells confirmed membrane association of TRDL-1, typical of TNF family members.