Exposure to biogenic aerosols such as airborne bacterium has hazardous effects upon human health. Mycobacterium tuberculosis (MTB) bacteria is a highly pathogenic microbe and arises large challenges to the enclosed laboratories for biosafety and the health of people working in them. Number concentration and size spectra of indoor aerosols were examined in the Animal Biosafety Level 3 (ABSL-3) laboratory located in Shanghai’s south suburb in July 2010. Mean particle concentration at center of the core room in dissecting experiment to MTB infected mouse (200 cm-3) was equivalent to feeding experiment to MTB infected mouse (200 cm-3), and roughly 2 times higher than background (91 cm-3). Mean particle size distribution at center of the core room exhibited a bi-peak feature under dissecting and feeding conditions, but a mono-peak feature under background condition. Because of dual negative air pressure and high frequency cleaning, MTB bacteria emitted from infected agents did not appear in the air of the preparation and core rooms. Airborne particle sampling demonstrated that MTB aerosol contamination was not detected in the indoor air. This result indicates that online monitoring for particle microphysical properties is one valuable approach to early warn and protect the safety of ABSL-3 laboratories.