The Fas/Fas ligand (FasL) interaction is pivotal in apoptosis-mediated regulation of the immune system. As such, it has relevance in areas of transplantation, gene therapy, AIDS, etc., all of which utilize the rhesus macaque as a preclinical animal model. In order to examine rhesus Fas/FasL, we cloned the rhesus FasL cDNA and have analyzed the function of the cloned gene. Our findings indicate that the rhesus FasL is highly homologous to the human but not the mouse (97% for human, 85% for mouse). In addition, soluble rhesus FasL can induce apoptosis, a property shared with the human soluble protein but not the mouse protein. The deduced protein sequence is 280 amino acids with a calculated Mr. of 31,646. Transfection of COS cells with the full-length cDNA yielded a 40 KD protein, which is in agreement with the size of human FasL. COS cells expressing rhesus FasL induced apoptosis in rhesus PHA blasts and human Fas+ CEM-6 cells. Thus, the cloned rhesus macaque FasL is functional and cross-reacts with human Fas. The cloned functional rhesus FasL cDNA will enable studies of its regulatory role in the nonhuman primate immune system.