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The research project described here studied the concentration of airborne endotoxin and aerobic gram-negative bacteria in 4 barns housing beef and dairy cattle. The goal was to determine whether a relationship existed between those wo air hygiene parameters and the following parameters: airborne dust, total aerobic bacteria concentration, airborne particle concentration, stable temperature and relative humidity.Species identification of the collected airborne gram-negative bacteria provided an overview of the aerobic gram-negative airborne bacterial flora in the studied animal houses.The airborne endotoxin concentration was measured in the inhalable dust fraction. Airborne bacteria were collected by using an Andersen Sampler. Species determination of isolated gramnegative bacteria was done by using the API systems from bioMerieux.Endotoxin concentrations measured in the stable air ranged from 8,0 to 721,9 EU/m³ air (EU = endotoxin units) in the cow houses. In the calf stable these values ranged from 101,6 to 205,9 EU/m³. In general the endotoxin content in the air was higher in dairy cow houses compared to the beef stables. Higher values were measured in each case in loose housing systems. Here endotoxin concentrations reached values that could present a potential health risk for the personnel. Sources for the airborne endotoxin were, in particular, the animal excrement, the sedimentation dust and feed stuff.The airborne aerobic gramnegative bacterial concentrations ranged from 1,2 to 102,5 cfu/ml air (cfu = colony forming units) in the cattle houses. These values were less than 1 % of the total aerobic bacterial count. The most significant sources of airborne gram-negative bacteria were the animals themselves and the sedimentation dust. Aerobic gram-negative bacterial concentrations in the air in the calf stable ranged from 12 to 1900 cfu/m³ . These high concentrations of gram-negative bacteria were related to certain procedures, such as cleaning and changing of the bedding material. The total bacterial count therefore did not present an accurate picture of the concentration of airborne gramnegative bacteria.The concentration of airborne endotoxin was not related to the concentration of inhalable dust or the number of airborne aerobic gram-negative bacteria. Further, the bacterial contamination of stable air was not related to the concentration of airborne dust. No relationship was found between airborne gram-negative bacteria and the stable temperature or the relative humidity.The gram-negative bacterial flora of a milieu has a significant influence on the measurable endotoxin concentration. The aerobic gram-negative airborne bacterial flora in the investigated cattle houses included bacterial species of the families Neisseriaceae, Enterobacteriaceae and Pseudomonadaceae. Airborne Neisseriaceae, primarily the species Acinetobacter lwoffii, were found quite frequently in cow houses. When cattle were maintained with bedding, Enterobacter agglomerans was the most prevalent Enterobacteriaceae, whereas a large percentage of E. coli were isolated when cattle were maintained without bedding. A number of different species of Pseudomonadaceae were found in the air of cattle houses. All bacterial species isolated from air in the stables were also found in various sources, including excrement, litter, feed and sedimentation dust.The fact that the endotoxin activity persists for a long time after bacterial cell death results in an accumulation of endotoxin in various sources. Due to the high stability of endotoxin in the environment, the inhalable endotoxin concentration appears to be a more suitable air hygiene parameter for determining health risks in stables than the airborne gram-negative bacteria concentration.