The lateral ventricle in adult mammalian brain is widely acknowledged as one of the areas that undifferentiated neural cells such as neural stem cells and neural progenitor cells inhabit. However, immunological aspects of neural stem cells in the lateral ventricle are still under debate. Here, we report the generation and characterization of a novel monoclonal antibody (mAb), called Namu mAb, which stains the subventricular zone in the lateral ventricle of adult mouse brain. Namu mAb reacted to the cells in the subventricular zone and never reacted to differentiated neural cells such as neurons and glial cells such as astrocytes and oligodendrocytes. Its reaction pattern for the subventricular zone and the neurospheres was similar to that of Nestin and glial fibrillary acidic protein mAbs. Namu mAb recognition molecule, Namu antigen, was a 50 kDa protein present in the cytoplasmic fraction of mouse brain, and its expression was clearly observed in neurospheres cultured in the presence of epidermal growth factor, but it was never or only weakly induced in the presence of basic fibroblast growth factor or leukemia inhibitory factor. Collectively, it is concluded that Namu mAb specifically reacts to undifferentiated neural cells in mouse brain.