Extracts from the mushroom Agaricus blazei Murill (AbM) are used extensively as a non-prescription remedy against cancer and infections, including hepatitis. We previously demonstrated a potent immunomodulating effect of a particular preparation on monocytes in vitro, and a protective effect on bacterial infections in mice. Here we report the effect on gene expression in peripheral blood cells from four chronic hepatitis C patients, using global (29 k) oligo-based, single channel microarrays. The viral load was slightly, but not significantly, decreased after 1 week of AbM treatment. The cytokine genes most strongly induced in vitro were not induced in vivo. The more notable changes in mRNA levels were related to genes involved in the G-protein coupled receptor signalling pathway, in cell cycling, and in transcriptional regulation. The results suggest that the β-glucans of the extract, which presumably are responsible for cytokine induction, did not readily enter the blood, while other components, such as substances proposed to have anticancer effects, were active in the blood.