Endotoxin has been widely recognized as an animal-production-related biological hazard and should ideally be removed from the exhaust air stream of an animal confinement building. The size and concentration of endotoxin at the building 's air exhaust constitutes essential information for designing and operating an endotoxin abatement process. In this study, PM10 and PM2.5 samples were seasonally collected at the air exhaust of 18 commercial animal confinement buildings (including torn turkey, laying hen, swine gestation, swine farrowing, swine weaning, and swine finishing), and the PM10 and PM2.5 associated endotoxin levels were determined using a kinetic chromogenic Limulus amoebocyte lysate (LAL) assay. The highest airborne endotoxin concentrations were detected in torn turkey buildings (geometric mean concentration [GM]: PM10 - 693.0 endotoxin units [EU]/m3 [19.62 EU/ft3]. PM2.5 - 78.4 EU/m 3 [2.22 EU/ft3]) while the lowest concentrations were found in swine gestation buildings (GM: PM10-63.9 EU/m3 [1.81 EU/ft3], PM2.5-20.5 EU/m3 [0.58 EU/ft3]). Poultry buildings had substantially higher airborne endotoxin concentrations than swine buildings. However, in terms of endotoxin concentration in PM (i.e., endotoxin per mass of PM, EU/mg), no significant difference was seen between poultry and swine or among dijferent building/operation types. PM10 samples had on average 169% ± 133% higher endotoxin concentrations in PM than their corresponding PM 2.5 samples, suggesting that endotoxin pollution was mainly contributed by coarse particles. Ambient temperature/season showed no significant effect on airborne endotoxin concentrations but had significant effects on endotoxin concentration in PM samples.