Objective Since most reports on bystander effect have been only concerned with radiation-induced damage, the present paper aimed at disclosing whether low dose radiation could induce a stimulatory or beneficial bystander effect. Methods A co-culture system containing irradiated antigen presenting cells (J774A.1) and unirradiated T lymphocytes (EL-4) was established to observe the effect of J774A.1 cells exposed to both low and high doses of X-rays on the unirradiated EL-4 cells. Incorporation of 3H-TdR was used to assess the proliferation of the EL-4 cells, expression of CD80/86 and CD48 on J774A.1 cells was measured with immunohistochemistry and flow cytometry, respectively. NO release from J774A.1 cells was estimated with nitrate reduction method. Results Low dose-irradiated J774A.1 cells could stimulate the proliferation of the unirradiated EL-4 cells while the high dose-irradiated J774A.1 cells exerted an inhibitory effect on the proliferation of the unirradiated EL-4 cells. Preliminary mechanistic Objective Since most reports on bystander effect have been only concerned with radiation-induced damage, the present paper aimed at disclosing whether low dose radiation could induce a stimulatory or beneficial bystander effect. Methods A co-culture system containing irradiated antigen presenting cells (J774A. 1) and unirradiated T lymphocytes (EL-4) was established to observe the effect of J774A.1 cells exposed to both low and high doses of X-rays on the unirradiated EL-4 cells. Incorporation of 3H-TdR was used to assess The proliferation of the EL-4 cells, expression of CD80 / 86 and CD48 on J774A.1 cells was measured with immunohistochemistry and flow cytometry, respectively. NO release from J774A.1 cells was estimated with nitrate reduction method. Results Low dose-irradiated J774A.1 cells could stimulate the proliferation of the unirradiated EL-4 cells while the high dose-irradiated J774A.1 cells exerted an inhibitory effect on the proliferation of the unirradiated EL-4 cells. Prelim inary mechanistic