Aluminum (Al) ion in acidic soils is known to be a major constraint in plant growth. Al binds to root apices and causes cell elongation inhibition immediately, and necrosis after longer exposure. To elucidate the mechanism of Al-induced cell elongation inhibition and cell death, suspension cultured cells of tobacco (Nicotiana tabacum L., cell line SL) and rice (Oryza sativa L.) have been used as a model system of the cells at apical root meristem. In tobacco cells, cell death process under Al exposure is strongly related to mitochondrial dysfuntion and reactive oxygen species (ROS) production, which was also observed in pea roots (Yamamoto et al., Plant Physiol. 128: 63-72 (2002) ; Yamamoto et al., Plant and Soil 255: 239-243 (2003); Kobayashi et al. Soil Sci. Plant Nutr. 50: 197-204 (2004)). In this study, we found that salicylic acid (SA) content was increased 5 to 10 times under Al exposure in tobacco cells. Increases in SA and ROS as well as a loss of growth capability concomitantly started at several hours after a start of Al exposure. SA is induced under several biotic and abiotic stresses. During pathogen-and-plant interactions, SA is reported to be an ROS inducer and to be involved hyper sensitive cell death process. Taken together, we concluded that SA seems to be a mediator of Al stress signal to cause ROS production and cell death. Compared to tobacco cells, rice cells were much more tolerant to Al, and the Al-induced ROS production was also lower in rice cells. Interestingly, compared to tobacco cells, SA content was higher under normal growth condition, but was reduced significantly under Al exposure in rice cells. These results suggest that the prevention of SA production seems to be involved in Al tolerant mechanism in rice cells.