In this study we evaluated the effects of the anti-microsporidial exospore monoclonal antibody 3B6, recognizing 3 Encephalitozoon species, Encephalitozoon intestinalis (Syn. Septata intestinalis), Encephalitozoon cuniculi, and Encephalitozoon hellem on microsporidial growth in vitro. Pre-treatment of spores for 24 h with mAb 3B6 resulted in 21-29% fewer infected host cells 4 days after inoculation of the cultures compared to cultures pre-treated with medium or an irrelevant isotype control mAb (P < 0.001). Fewer intracellular spores (1.2 +/- 0.2) in infected cells were found when mAb 3B6 was present in cultures compared to cultures with medium alone (4.3 +/- 0.8) or an irrelevant isotype control mAb (4.2 +/- 0.9; P < 0.001). This decrease appeared not to be dependent on time of exposure, mAb concentration, or presence of complement. It is concluded that antibodies, particularly those directed to potential neutralizing-sensitive epitopes on spores, may have a role in the control of microsporidial growth in vitro.