Concentrations of extractable and insoluble tannins (condensed proanthocyanidins) were determined in bracts as functions of cultivar, growing location (MS, CA, TX), year (1982-84), and time of harvest (before and after frost), and in mill dusts as functions of Particle size, year, and growing location. Total tannin concentrations and solubility in 80% acetone decreased following frost and with browning of bracts. The cultivar 'Acala SJ5' contained significantly more tannin than 'Deltapine 61' or 'GSA 71.' Large variations in tannin concentrations occurred among years and growing locations with lowest concentrations observed in 1983 and in MS-grown cotton. Tannin concentrations were highest (4.5-10.5%) in medium size (38-150 m) dust particles; 1.89-3.11% were found in fine particles collected from animal and human exposure rooms. In general, tannin concentrations were negatively correlated with microbial or endotoxin content of dusts, indicating microbial breakdown of these chemicals in dead cotton tissues.