Fu-Ling, the sclederma of Poria cocos (Schw.) Wolf, has long been used as a sedative and diuretic. However, data in this report suggest that Fu-Ling is a potential suppressor of cytokine secretion from human peripheral blood monocytes under in vitro condition. Monocyte culture medium containing 10% of Fu-Ling extract significantly inhibited secretion of TNF-alpha, IL-beta, IL-6 and GM-CSF from the monocyte monolayer. However, as Fu-Ling extract content was gradually reduced, cytokine secretion was augmented in comparison with the cytokine secretion in drug-free controls. This augmentative effect resulted from the trace amount (1.24 ng/ml in 0.62% of Fu-Ling extract) of lipopolysaccharide (LPS) which contaminated the Fu-Ling extract during the preparation process, since TNF-alpha, IL-1 beta and IL-6 secretion induced by 0.62% Fu-Ling extract could be significantly inhibited by polymyxin B, an LPS inhibitor. Furthermore, the amounts of TNF-alpha IL-1 beta and IL-6 induced by 1 ng/ml of LPS without the presence of drug were more than that induced by 0.62% of Fu-Ling extract. Thus, cytokine secretion induced by LPS contamination (1.24 ng/ml) in the Fu-Ling extract was partially suppressed by 0.62% of the Fu-Ling extract itself. GM-CSF secretion in the medium containing 0.62% of Fu-Ling extract was not induced by LPS since: a) GM-CSF induced by 0.62% Fu-Ling extract could not be inhibited by polymyxin B; b) LPS at 1 ng/ml showed no activity indicating induction of GM-CSF secretion.