Impairment of T-cell function is a consistent observation in burned patients. Concomitant with this impairment is an increase in serum factors which inhibit interleukin-2-mediated T-cell functions. These factors are heat labile and do not behave like endotoxins. Nonetheless, treatment of burned patients with endotoxin-neutralizing regimens of polymyxin B reduces the levels of these factors, suggesting that they are generated in response to endotoxin exposure. In addition to factors which inhibit Il-2 responses burn serum contains increases of circulating soluble, cell-free Il-2 receptors. However, the level of Il-2R is not altered by polymyxin B treatment and does not appear to be a direct result of endotoxin exposure. These observations suggest that multiple causes contribute to T-cell impairment in burned patients.