A colorimetric assay was used for quantitating cytotoxic effect on A549 and VERO cells. The assay was conducted by incubating cell cultures with sterile extracts of cotton dust, suspension of pure endotoxin or bacterial supernatant in microtiter plates for 2h and 24h. In another series of experiments the cells were exposed to cotton dust extracts or bacillus supernatant in the presence of 10 or 100 mg endotoxin/ml. After exposure Tetrazolium slat was added, and 4h later solubilizer was added 16h prior to reading of the results in an ELISA microplate reader at 570 nm. Absorption of formazan was directly related to viable cell number and their growth. This assay detected an effect of the exposure being strongest after 24h of exposure. The concentrations needed to elicit an effect was 2 logs higher for the bacterial supernatant than for cotton dust, when they were related to the daily intake during 8 hours of work in a cotton mill. No effect was elicited by endotoxin, either alone nor in combination with other exposures. Thus we conclude, that in this model endotoxin has no effect, and the effect of cotton dust cannot be explained by the effect of Bacillus supernatants alone. Other components of the dust must also be involved in the cytotoxicity, and these might be tannins that have been shown to elicit cytotoxic effects in other studies.