The possible mechanism of a radiationinduced bystander response was investigated by using a high-LET heavy particle microbeam, which allows selected cells to be individually hit with precise numbered particles. Even when only a single cell within the confluent culture was hit by one particle of 40Ar (～1260keV/μm) or 20Ne (～380 keV/μm), a 1.4-fold increase of micronuclei (MN) was detected demonstrating a bystander response. When the number of targeted cells increased, the number of MN biphasically increased; however, the efficiency of MN induction per targeted cell markedly decreased. When 49 cells in the culture were individually hit by 1 to 4 particles, the production of MN in the irradiated cultures were ～2-fold higher than control levels but independent of the number and LET of the particles. MN induction in the irradiatedculture was partly reduced by treatment with DMSO, a scavenger of reactive oxygen species (ROS), and was almost fully suppressed by the mixture of DMSO and PMA, an inhibitor of gap junctional intercellular communication (GJIC). Accordingly, both ROS and GJIC contribute to the above-mentioned bystander response and GJIC may play an essential role by mediating the release of soluble biochemical factors from targeted cells.-Shao, C., Furusawa, Y., Kobayashi, Y., Funayama, T., Wada, S. Bystander effect induced by counted high-LET particles in confluent human fibroblasts: a mechanistic study. FASEB J. 17, 1422-1427 (2003)