We have previously shown that cimetidine is capable of reducing the clastogenic effect of gamma-rays. In this research the radioprotective property of this drug was examined against low doses of fast neutrons using the micronucleus assay. Swiss albino male mice (12 weeks old) were irradiated by fast neutrons emitted from a 241Am-9Be source. The absorbed doses were 1.5, 2.25, 3.375 and 5.06 cGy at a dose rate of 0.718 cGy/h. Two hours prior to neutron irradiation mice were treated by cimetidine at a concentration of 15 mg/kg body weight injected i.p. Mice were sacrificed by cervical dislocation at different post-irradiation times (24, 48 and 72 h). The results obtained show that the frequency of neutron-induced micronuclei in polychromatic erythrocytes (PCEs) is significantly higher than those of control groups (P < 0.05) at the neutron doses used in these experiments. Moreover, cimetidine effectively reduced (1.5-2-fold) the frequency of micronuclei in PCE (P < 0.05). These results show that cimetidine can protect bone marrow cells against clastogenic effects of low dose fast neutrons and hence high linear energy transfer (LET) radiation. The mechanism by which cimetidine reduces the clastogenic effects of fast neutrons is not fully understood. It might act through a free radical scavenging mechanism associated with the amplification of the glutathione system.