A monoclonal antibody (mAb) 3C2, against an exospore protein of the microsporidium Nosema bombycis (N. bombycis) was prepared, and its effects on microsporidial germination and reproduction in vitro were studied. MAb 3C2 was effective in inhibiting the germination and subsequent infection of Bombyx mori cells compared to the control mAb. The antigen was isolated by 2-dimensional gel electrophoresis. Immunoblotting revealed it to be an 84 kDa protein corresponding to pI (7.2) on the 2-D gel. The present results suggest that the antibodies can be used for diagnostic purposes and for developing new therapeutic strategies in controlling microsporidian diseases.