['In a recent study, we showed that unirradiated cells, when they are in the presence of cells irradiated with gamma-rays, are characterized by enhanced cell growth (Cytometry 2003;54A:1-7). However, the mechanisms and factors involved in the proliferative response of bystander cells are largely unknown. The aim of the current work was to investigate the possible role of spatial proximity of cells, including gap junctional intercellular communication (GJIC), in transmitting proliferation signals from cells irradiated with gamma-rays to unirradiated cells (bystander cells).', 'Confluent monolayers of rat liver epithelial cells (WB-F344) were irradiated with 137Cs gamma-rays at doses ranging between 0.5 and 10 Gy. The impact of cell proximity on the proliferative response of bystander cells was determined by co-culturing different densities of irradiated and unirradiated cells (ratio 1:1) followed 24 h later by two-scheme flow cytometric analysis of their proliferation. To determine whether soluble extracellular factors play a role in this response, irradiated cells were plated together with unirradiated cells at the same ratio (1:1), but they were not allowed to directly contact unirradiated cells by using porous low-protein binding polyethylene terephthalate membranes. After 24 h, the numbers of unirradiated cells in the co-culture with irradiated cells were compared with the numbers of unirradiated cells in the control (unirradiated cells co-cultured with unirradiated cells). To investigate the possible involvement of GJIC in mediating the proliferative response in the bystander cells, the proliferation status of unirradiated GJIC-incompetent WB-aB1 cells was compared with that of unirradiated WB-F344 cells that were GJIC competent. In all of the protocols, the two populations of co-cultured cells were distinguished by labeling one population with fluorescent cell tracers, such as membrane-resident or CFDA SE (carboxy) fluorescein diacetate, succinimidyl ester that metabolizes intracellularly.', 'Unirradiated cells that were co-cultured with irradiated cells, but were not allowed to directly contact them, did not show any changes in the proliferation rate compared with that of unirradiated cells in the control. Unirradiated cells that were co-cultured as a mixture with irradiated cells showed enhanced and statistically significant proliferation, particularly when they were plated together more densely. The proliferation rate of bystander WB-aB1 cells was apparently higher than that of bystander WB-F344 cells; however, the difference was statistically insignificant.'] Is cell proximity a prerequisite for the proliferative response of bystander cells co-cultured with cells irradiated with gamma-rays?