Environmental and other factors contributing to bioaerosol exposures in dairy barns were analyzed. Three hour ambient air samples were collected from 23 dairy barns in the Midwest USA that used fodder prepared from crops grown in 1991, a season of normal rainfall, rainfall averaging 25.5 centimeters (cm) (dry barns) and from 24 barns in the region that used fodder prepared from crops grown in 1993, a season of heavy rainfall and flooding, rainfall averaging 63.0cm (wet barns). Samples were analyzed for yeasts, molds, mesophilic and thermophilic bacteria, total dust, and carbon-dioxide (124389). Dairy management practices such as type of ventilation, bulk food and bedding used and estimates of moisture content, distribution of lime for dewetting and disinfection, and animal density were determined. Time weighted geometric mean yeast, mold, mesophilic and thermophylic bacteria exposures across all barns were 1.8x10(4), 0.8x10(4), 81.1x10(4), and 0.4x10(4) colony forming units per cubic meter, respectively. Among individual barns, microorganism concentrations varied by two to three orders of magnitude, but did not vary between dry and wet barns. Barns with mixing ventilation had three to eight fold higher thermophilic concentrations than other barns. Tunnel ventilation reduced carbon- dioxide concentrations by 50%. Barns where hay was bulk feed had mold concentrations five times higher than silage use or no bulk feed. Fresh straw used as bedding, with highest moisture, produced highest mean concentrations of yeasts, molds, mesophilic bacteria, and total dust were compared to day old straw or no straw. Use of lime was highly correlated with use of fresh straw. Multiple regression analysis showed that number of cows in the barn, temperature during sampling, and use of a supply ventilation system were significant determinants of airborne yeast concentrations. Feeding dry grain and hay were associated with mold concentrations. Feeding moist grain was associated with mesophilic bacteria concentrations, and feeding silage was associated with thermophilic bacteria concentrations. The authors conclude that efforts to reduce bioaerosol exposures in dairy barns should focus on ventilation and storage moisture of feed grains.