Twelve Bacillus, 8 Enterobacteriaceae and 3 Pseudomonas strains isolated from the air of Danish cotton mills have been tested in Vero and A549 cells for production of toxic effects. Similar effects had been observed with strains of the same species isolated from the air of English cotton mills. Among the Danish strains, 3 Gram-positive and 1 Gram-negative also produced an effect on the cell walls of both cell-lines. The results support the hypothesis that bacterial toxins may play a role in the aetiology of byssinosis since the cell-line derived from human pulmonary epithelium appeared to be even more susceptible to the toxin-producing strains than the Vero cells.