Endotoxin (ET) is the outer membrane component of gram-negative bacteria and secreted by destruction of bacterial cell. It is ubiquitous in living environment and elicits strong immune responses such as inflammation, fever, septic shock etc. when get into the blood. It also known as lipopolysaccharides (LPS) and lipoglycans, has been ascribed as allergen on the hygiene hypothesis.<br> Since most of the eubacteria are gram-negative and ET is also the specific substance produced from gram-negative bacteria, it is reasonable premise that ET concentration can serve as a representative index to bacterial pollution.<br><br> The aim of this study is to prove the benefit of measuring ET as a substitute for the traditional cultural method to investigate the bacterial pollution in indoor air.<br><br> The followings were reported in this article.<br> 1) Interference tests to gelatinization of lysate reagent in order to select the air sampling filter and preservation method<br> 2) Field survey results on airborne ET and bacteria concentrations in indoor air at office buildings and elderly facilities<br> 3) Correlation analysis between ET and bacteria concentration<br><br> As a result, it was obvious that ET concentration tended to be high in elderly facilities than in offices, and the same tendency was not observed from airborne bacterial concentrations measured with the cultural method. In elderly facilities, there were several rooms that indicated evidently higher ET concentrations and a few of them observed repeatedly in the winter seemed to be associated with domestic humidifiers and/or a drop in ventilation amount.<br> I/O ratios of bacterial concentrations in offices differed significantly with those of ET as “I/O ratio≧1” was observed more frequently, while were not marked by a difference in elderly facilities. Numerical values of I/O ratios of ET concentrations had a tendency to fall behind those of bacterial concentrations.<br> “Outdoor air” of elderly facilities and “indoor air” of offices revealed the moderate correlation between ET and bacterial concentrations. Meanwhile, the correlation between the two methods was derived from all data of elderly facilities. For whole values measured, however, the moderate correlation was found only in outdoor air. Though the moderate correlations could be obtained partially between ET and bacterial concentrations, the definite correlation could not be obtained as a whole in all the measurement results.<br> Determination method of ET has a high performance and reliability in terms of quantitative accuracy and reproducibility. Even if there are several limitations to be resolved, determining ET might be useful to investigate the bacterial contamination from the viewpoint of practical use in field survey and analysis convenience.<br><br> Further studies should be also performed to comprehensively understand the correlation between airborne ET and bacterial concentrations, and one example for possible consideration might be a genetic sequencing method.