Airborne microorganism level is an important indoor air quality indicator, yet it has not been well documented for laying-hen houses in the United States. As a part of the Coalition for Sustainable Egg Supply (CSES) environmental monitoring project, this study comparatively monitored the concentrations and emissions of airborne total and Gram-negative (Gram(-)) bacteria in three types of commercial laying-hen houses, i.e., conventional cage (CC), aviary (AV), and enriched colony (EC) houses, over a period of eight months covering the mid and late stages of the flock cycle. It also delineated the relationship between airborne total bacteria and particulate matter smaller than 10 mu m in aerodynamic diameter (PM10). The results showed airborne total bacteria concentrations (log CFU/m(3)) of 4.7 +/- 0.3 in CC, 6.0 +/- 0.8 in AV, and 4.8 +/- 0.3 in EC, all being higher than the level recommended for human environment (3.0 log CFU/m(3)). The much higher concentrations in AV arose from the presence of floor litter and hen activities on it, as evidenced by the higher concentrations in the afternoon (with litter access) than in the morning (without litter access). The overall means and standard deviation of airborne total bacteria emission rates, in log CFU/[h-hen] (or log CFU/[h-AU], AU = animal unit or 500 kg live weight) were 4.8 +/- 0.4 (or 7.3 +/- 0.4) for CC, 6.1 +/- 0.7 (or 8.6 +/- 0.7) for AV, and 4.8 +/- 0.5 (or 7.3 +/- 0.5) for EC. Both concentration and emission rate of airborne total bacteria were positively related to PM10. Gram(-) bacteria were present at low concentrations in all houses; and only 2 samples (6%) in CC, 7 (22%) samples in AV, and 2 (6%) samples in EC out of 32 air samples collected in each house were found positive with Gram(-) bacteria. The concentration of airborne Gram(-) bacteria was estimated to be <2% of the total bacteria. Total bacteria counts in manure on belt (in all houses) and floor litter (only in AV) were similar; however, the manure had much more Gram(-) bacteria than the litter. The results point out the need to mitigate airborne total bacteria in laying-hen houses, especially in AV houses.