Sepsis induces extensive apoptosis of lymphocytes, which may be responsible for the profound immune suppression of the disorder. Two potential pathways of sepsis-induced lymphocyte apoptosis, Fas and p53, were investigated. Lymphocyte apoptosis was evaluated 20-22 h after sepsis by annexin V or DNA nick-end labeling. Fas receptor-deficient mice had no protection against sepsis-induced apoptosis in thymocytes or splenocytes, p53 knockout mice (p53(-/-)) had complete protection against thymocyte apoptosis but, surprisingly, had no protection in splenocytes, p53(-/-) mice had no improvement in sepsis survival compared with appropriately matched control mice with sepsis. We conclude that both p53-dependent and p53-independent pathways of cell death exist in sepsis. This differential apoptotic response of thymocytes vs splenocytes in p53(-/-) mice suggests that either the cellular response or the death-inducing signal is cell-type specific in sepsis. The fact that p53(-/-) lymphocytes of an identical subtype (CD8- CD4+) were protected in thymi but not in spleens indicates that cell susceptibility to apoptosis differs depending upon other unidentified factors.