AbstractA large number of different chemical compounds can be found in the air of densely populated animal production houses. These compounds may influence animal performance and health. Although knowledge of the biological activity of such compounds is limited in these situations, bioassays with animals are expensive, complicated to perform and evaluate. At the same time there is an increasing interest in simple and reliable test methods avoiding the use of laboratory animals. Microbial short‐term assays offer many advantages, particularly for screening purposes.The recently introduced Microtox assay was used to test the toxicity of eight single air contaminants, 13 two‐compound mixtures, and some extracts from aerosols and dust from a confined pig fattening house. p‐Cresol was the most toxic with an EC50(15 min) value of 0.015 mM followed by m‐cresol (0.074 mM), butyric acid (0.115 mM), dimethylamine (0.156 mM), formaldehyde (0.25 mM), phenol (0.46 mM), butanol (32 mM), and acetone (258 mM). The coefficients of variation for five replicates of these compounds ranged from about 5% (m‐cresol) to 24% (dimethylamine).The two‐compound mixtures showed mainly synergistic or antagonistic effects. The investigation of the aerosol extracts showed that increasing aerosol amounts also increase the necessary dilution ratio to reach the EC50(15 min) value. Futhermore, it was observed that the dust extract can stimulate the growth ofPhotobacterium phosphoreum(as determined by plate counts) although the bioluminescence is ceased. Apparently the inhibition of the light output of the bacteria is not necessarily accompanied by a loss o