Brain microbleeds often occur in Alzheimer's disease patients. Our previous studies have demonstrated that iron contributes to brain injury following intracerebral hemorrhage. This study investigated the effect of iron on amyloid β (Aβ)-mediated brain injury. There were two parts to this study. In first part, rats received an intracaudate injection of saline, iron, Aβ 25-35 or iron+Aβ 25-35. In the second part, rats received intracaudate injection of iron+Aβ and were treated with saline or cystamine, an inhibitor of transglutaminase. Rats were killed after 24 hours for brain edema measurement. DNA damage, neuronal death and tissue-type transglutaminase (tTG) expression were also examined. We found that brain water content in the ipsilateral caudate was higher (p<0.05) in rats injected with iron+Aβ than with iron, Aβ or saline. Combined iron+Aβ injection also resulted in more severe DNA damage (both single- and double-strand; p<0.01) and more Fluoro-Jade C staining (p<0.05). Expression of tTG increased markedly in the iron+Aβ group (p<0.05) and treatment with a tTG inhibitor reduced brain edema (p<0.05) and reduced degenerating neurons (124±25 vs. 249±50/mm(2) in vehicle-treated group, p<0.05). These results suggest that increased brain iron from microbleeds may exaggerate brain Aβ toxicity and that tTG is involved in the enhanced toxicity.