Possible toxic effects of multi-walled carbon nanotubes (MWCNTs) on plant cells were investigated. Suspension rice cells (Oryza sativa L) were cultured with MWCNTs; reactive oxygen species (ROS) increased and cell viability decreased were observed. When ascorbic acid, a primary antioxidant, was introduced into the culture suspension, the ROS content decreased and cell viability increased. Transmission electron microscopy revealed individual tubes in contact with the cell walls. The suspension rice cells with individual MWCNTs at their cell wall seemed to undergo a hypersensitive response, namely the ROS defense response cascade, which is sufficient to prevent microbial pathogens from completing their life cycle.