The cytotoxic effect of nitric oxide (NO) on mouse T lymphocytes was investigated. Freshly isolated T lymphocytes from mouse spleen were incubated with NOR, a novel NO releasing agent, at different doses. After incubation for 4 h, apoptotic cell death was observed in both NOR-treated T lymphocytes and controls as judged by the appearance of DNA laddering in agarose gel electrophoresis, but a quantitative analysis by flow cytometry indicated that the high level of exogenous NO (500 micrograms ml of NOR) could promote apoptosis in T lymphocytes as compared with controls (30% vs. 8%). After 8 h, NO promoted apoptosis of T lymphocytes in a dose-dependent manner. This study indicated that NO might be one of the factors which regulate this life of T lymphocytes in vivo.