Endotoxins, cell-wall fragments of gram-negative bacteria, are found in various work environments and first measurements have been made in general indoor environments. Endotoxins cause an inflammatory response of the respiratory tract. The response is mediated by the proinflammatory cytokines IL-1, IL-6, IL-8, and TNF-alpha and gives rise to general symptoms (fever, headache, malaise), respiratory symptoms (tightness of chest, dry cough), and lung function decrements. In the work environment endotoxins have been identified in all environments which produce similar symptoms. The qualitative results of experimental and epidemiological studies agree well. The related question whether endotoxins are the biologically active component of organic dust cannot yet be answered because of the gap between the concentration of lipopolysaccharides and of endotoxins necessary to induce the same quantitative effect. Different possible explanations are discussed. Endotoxins are also found in the general environment, especially indoors. Their health relevance needs to be assessed in more detail, especially in subjects with bronchial hyperreactivity.