In this paper it is shown that incubation with 2 mM caffeine enhanced significantly the MN （micronucleus） formation in both the 1 cGy α-particle irradiated and non-irradiated by- stander regions. Moreover, caffeine treatment made the non-irradiated bystander cells more sensitive to damage signals. Treated by c-PTIO（2-（4-carboxy-phenyl）-4,4,5,5-tetramethyl-imidazoline- 1-oxyl-3-oxide）, a nitric oxide （NO） scavenger, the MN frequencies were effectively inhibited, showing that nitric oxide might be very important in mediating the enhanced damage. These results indicated that caffeine enhanced the low dose m-particle radiation-induced damage in irradiated and non-irradiated bystander regions, and therefore it is important to investigate the relationship between the radiosensitizer and radiation-induced bystander effects （RIBE）.
