Mice over-expressing a human mutation of Cu(2+)/Zn(2+) superoxide dismutase (SOD1) provide a model of amyotrophic lateral sclerosis. Using tomato lectin histochemistry, we analyzed microglia in the facial nuclei of SOD1(G93A) transgenic mice in the late stage of disease. In these animals, microglia was markedly activated, and ensheathed facial motoneurons as observed in wild-type mice 1 week after nerve transection. In the axotomized facial nucleus of transgenic mice at the same time point, microglia activation was enhanced and exhibited phagocytic features. The findings show that in the facial nucleus microglial cells react to motoneuron disease caused by the SOD1 mutation and to axotomy-induced damage of facial motoneurons.