In the 1990 season, open cotton bolls were collected weekly during the growing season and the cotton was hand ginned and processed through a Microdust & Trash Monitor (MTM). Cotton dust was collected on filters that were later assayed for bacterial population. The amounts of cotton dust and associated bacterial populations increased during the growing season. The highest correlation with cotton dust potential over the season was with cumulative rainfall. This study was repeated for the 1992 season and the results are reported. The trend observed was that endotoxin potential of respirable cotton dust and of cotton lint increased during the growing season and this increase was highly correlated with cumulative rainfall (r=0.83 and r=0.91, respectively). This work relates to the etiology (cause) of byssinosis.