The LPS stimulated release of Interleukin-1 (IL-1) is thought to be the reason for the mill fever seen in cotton mill workers. This study provides the evidence that cotton bract tannin also has a potential of IL-1 induction, and thus could be an additional etiologic factor in the development of mill fever. Human monocytes were cultured alone or with peripheral blood T-lymphocytes, and stimulated with 100 µg/ml of tannin. Control cultures included unstimulated cells, and cells challenged with other IL-1 inducers: Con A and LPS from E. coli or E. agglomerans. IL-1 was measured in culture supernatants 24 hours after the culture initiation by the use of an ELISA or an RIA. The results showed that tannin stimulated monocytes to secrete IL-1 in a manner similar to Con A, i.e. substantially more cytokine was measured in the supernatants of monocyte-T-lymphocyte co-cultures than in the cultures of monocytes alone. Endotoxin from E. coli was less effective than the endotoxin from E. agglomerans in IL-1 induction, but they both acted directly on monocytes. Contaminating endotoxin present in the tannin preparation accounted for the majority of the IL-1 released from monocytes alone stimulated with tannin, but only 20% of the IL-, released from tannin stimulated monocyte-T-lymphocyte co-cultures. The results showed that tannin itself has IL-1 inducing ability. The dose-response studies showed that the extent of IL-1 release was dependent on tannin dose and that increased levels of monocyte produced IL-1 preceded the increase in tannin stimulated T-lymphocyte proliferation.