The action of the antibiotics present in deal sawdust on the growth on Czapek‐Dox agar of cellulose‐decomposing fungi has been examined.Stachybotrys atraandChaetomium indicumwere strongly inhibited by substances in cold‐water extracts, butC. globosumonly slightly so. The extracts also contained material which stimulated the growth ofC. globosumbut not that of the other two fungi. The formation of perithecia byC. indicumandC. globosumwas also stimulated by the extract. There was no marked inhibition or stimulation of the growth ofAspergillus terreus, A. fumigatus, or three species ofPenicilliumby the ext