A colorimetric assay is used for quantitating cytotoxic effect on human pneumocytes (A549) and monkey kidney cells (VERO). The assay is conducted by incubating cell cultures with sterile extracts of cotton bract, suspension of pure endotoxin from Enterobacter agglomerans (LPS) or bacterial supernatant in microtiter plates for 2h and 24h. After exposure formazan is added, and 4h later solubilizer is added 16h prior to reading of the results in an ELIZA microplate reader at 570 nm. Absorption of formazan is directly related to viable cell number and their growth rate. Only crude cotton dust and extracts from B. cereus elicited an effect on the cell cultures. We find a similar response in the two cell lines. However, the monkey kidney cells are more susceptible to the extracts than human pneumocytes. After a few hours we find morphological changes in the cell monolayer without a cytotoxic reaction. After 24 hours we find a significant cytotoxic effect. No effect is seen after exposure to LPS, glucans, FMLP or tannin. LPS has no adjuvant effect in this assay when tested with crude cotton dust, tannin or B. cereus. The lack of reaction to compounds other than crude cotton and B. cereus indicates that the cytotoxic assay detects something different from the stimulation seen in macrophages with subsequent release of cytokines. It is not known if this cytotoxic reaction is involved in the pathogenesis of the respiratory symptoms observed among the exposed. However, we are working with other reactions to study the timing of the response after exposure to organic components.