The aim of the study was to compare the pulmonary toxicity of cotton dusts with their microbial and chemical content to enable possible assessment of factors contributing to the pulmonary toxicity. The three dusts had equivalent toxicity, measured by changes in pulmonary function, and equivalent endotoxin content. However, there were differences in the Corynebacter and tannin contents of the dusts. The results supported contribution of endotoxin, not tannin, to the acute respirator response.