CD95-dependent lysis of target cells by CD4+ cells raises important questions of specificity and its potential role in inflammation. Much of what we know about CD95L-dependent killing is in fact bystander killing because the T cells have been previously activated by pharmacologic reagents. In an effort to better understand its significance in a more physiologic setting, we have examined bystander lysis by Th1 cells stimulated by Ag and APC in a murine model system. We find that bystander lysis is readily stimulated by M phi APC but not some epithelial lines, even though they themselves are killed in a CD95-dependent process while serving as APC. Bystander activity requires close proximity between the T cell, the APC, and the bystander target because bystanders are not lysed during coculture in Transwells.