Supernatants prepared from 17 strains of Bacillus species, 4 strains of Pseudomonas species, 2 Enterobacter agglomerans and 2 Flavobacterium strains have been tested for toxic effects A549 cells. The cell line was sensitive to all the Bacillus species tested except B. brevis in varying degree and also at 20 hours to the gram-negative bacteria. Since this cell-line is derived from pulmonary tumour-cell epithelium, it suggests that toxins from air-borne bacteria may be an aetiological factor in byssinosis.