The aqueous extract of the shiitake mushroom was found to decrease IL-1 production and apoptosis in human neutrophils, as measured by ELISA and ﬂow cytometry respectively. It was found to increase IL- 1 production and apoptosis in the U937 monocytic cell line. The extract showed no signiﬁcant effects on the superoxide production of both neutrophils and U937 cells, as measured by chemiluminescence. The extract was further separated into high and low molecular weight components, and it was found that the low molecular weight component retained the activity of the whole extract. This further suggests that the active substance is a novel compound distinct from lentinan, a well-studied high molecular weight antitumour agent found in shiitake. Copyright # 1999 John Wiley & Sons, Ltd.