Nitric oxide (NO^•) is mutagenic and, under appropriate conditions of exposure, also induces apoptosis in many in vitro and in vivo experimental models. Biochemical and cellular mechanisms through which NO^• induces apoptosis are incompletely understood, but involve p53/mitochondria-dependent signaling pathways. In this study, we exposed human lymphoblastoid cells harboring either wild-type (TK6 cells) or mutant p53 (WTK-1 cells) to NO^•, delivered by diffusion through Silastic tubing. Cells were exposed for 2 h at constant rates of 100–533 nM/s, similar to levels estimated to occur in vivo in inflamed tissues. DNA double-strand breaks and fragmentation detected 8–48 h after NO^• treatment were more extensive in TK6 cells than in WTK-1 cells, whereas NO^•-induced mutant fractions in both HPRT and TK1 genes were significantly lower in TK6 cells than in WTK-1 cells (P < 0.01–0.05). Treatment of TK6 cells with NO^• caused extensive apoptosis, but this response was delayed and greatly reduced in magnitude in WTK-1 cells. Mitochondrial membrane depolarization and cytochrome c release were induced in both cell types. However, elevation of apoptotic protease-activating factor-1 (Apaf-1) protein and reduction of X-chromosome-linked inhibitor of apoptosis (XIAP) protein were observed only in TK6 cells. These results indicate that p53 status is an important modulator of NO^•-induced mutagenesis and apoptosis, and suggest that levels of the Apaf-1 and XIAP proteins, but not mitochondrial depolarization and cytochrome c release, are regulated by p53 in these human lymphoblastoid cells. Thus, Apaf-1 and XIAP may play important roles in the regulation of p53-mediated apoptotic responses.