Water extracts of the mycelial culture and fruiting bodies of Agaricus blazei Murill were fractionated by ethanol precipitation using various ethanol concentrations. Original water extracts from mycelia (Fraction A-0) and fruiting bodies (Fraction B-0) induced TNF-α secretion by macrophages derived from rat bone marrow. Fractions B-4 and B-5 obtained from ethanol precipitation of fruiting bodies using 44% and 50% ethanol, respectively, and Fraction B-6 obtained from the supernatant at 50% ethanol markedly induced TNF-α secretion. Similar effects were observed in IL-8 secretion by macrophages. Regarding nitric oxide (NO), Fraction B-5 induced a significant increase in NO secretion and Fractions B-4 and B-6 induced slightly NO secretion. Northern blot analysis showed that the increases in cytokine- and NO secretion were due to an increase in cytokine mRNAs or NO synthase mRNA. Therefore, it is concluded that Agaricus blazei Murill components which activate macrophages result in the induction of cytokine- and NO secretion in vitro.