Humanin (HN) was originally identified as an endogenous peptide that protects neuronal cells from apoptosis by mutant Alzheimer's disease genes. This 24-residue peptide has been recently shown to suppress apoptosis by interfering with activation of Bcl-2-associated X protein (Bax) in cytosol. In the present study, we showed that HN increases ATP levels in human lymphocytes, muscular TE671 cells, and neural SKN-MC cells, and protects these cells from serum deprivation-induced apoptosis. The suppressed apoptotic death of serum-deprived cells would be explained by the anti-Bax effect of HN; however, HN also increased ATP levels of serum-supplemented cells (non-apoptotic cells), in which Bax is likely to be inactive. This result suggests the presence of a certain mechanism independent of Bax inactivation to increase ATP levels of cells under non-apoptotic condition. By treatment with HN, the ATP levels of lymphocytes from patients with mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) associated with A3243G mutant mtDNA were increased as well, suggesting that HN is able to prevent cells in MELAS from falling into ATP deficiency. Our quantitative PCR findings indicated that the HN-induced increase in ATP may not be a consequence of mitochondrial proliferation, because HN rather suppressed mtDNA replication. This suppression may be important in the treatment of affected cells in MELAS, since the mutant mtDNAs that increase during compensatory mtDNA replication for ATP deficiency cause excessive formation of reactive oxygen species, leading to further energy crisis. We thus propose that HN, which increases cellular ATP levels without inducing mtDNA replication, may be suited for the treatment of MELAS.