Factors influencing the penetration of bacterial aerosols through respirator filters and surgical masks were examined. An unspecified number of surgical masks and respirator filters including dust/mist (DM), dust/fume/mist (DFM), and high efficiency particulate air (HEPA) filters were challenged with aerosolized Mycobacterium- abscessus, Staphylococcus-epidermides, and Bacillus-subtilis in a custom built enclosed nebulization/exposure system. The bacterial aerosols used were chosen to represent a range of shapes and sizes typical of infectious bacteria: M-abscessus (rod shaped acid fast), S-epidermides (spherical), and B-subtilis (rod shaped, spore forming). The experiments were conducted at flow rates of 45 and 85 liters per minute (l/min) and 30 and 70% relative humidity (RH). Aerosol particle concentrations were measured upstream and downstream from the filters or masks using a total particle direct reading spectrometer and a viable particle cascade impactor. The extent to which the aerosols penetrated each filter and mask was calculated from the particle concentration data. Similar experiments were performed using polystyrene latex spheres (PLS) for comparison. Across all bacterial aerosol exposures, the surgical masks showed the greatest aerosol particle penetration, followed by the respirator DM, DFM, and HEPA filters. Greater penetration occurred at the 85l/min flow rate than at the 45l/min rate, as expected. By bacterial species, M-abscessus and B-subtilis penetrated to a significantly greater extent at the higher flow rate, whereas the extent of penetration of S-epidermides was similar at both air flow rates. Greater penetration occurred at the lower RH than at the higher RH; however, the difference was statistically significant only for M-abscessus. Across all filter types, PLSs were the most penetrating followed by M-abscessus, B-subtilis, and S- epidermides. When compared to their predicted penetrability based on their aerodynamic diameters, the PLSs and M-abscessus aerosol agreed with theory; however, S-epidermides had been predicted to be more penetrating than B-subtilis. The authors conclude that aerodynamic diameters may not be the best parameter for predicting the penetrability of bioaerosols. The data suggest that electrostatic and mechanical forces may be responsible for the observed penetrability of the bioaerosols through the tested filters and masks.