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Eight previously undescribed sesquiterpenoids, tremutins A-H ( 1 - 8 ), together with three known ones ( 9 - 11 ), were isolated from cultures of the basidiomycetes Irpex lacteus . Structures of the new compounds together with absolute configurations were elucidated on the basis of extensive spectroscopic methods, as well as single-crystal X-ray diffractions and equivalent circulating density calculations. Compounds 1 and 2 possess an unusual 6/7-fused ring system that might be derived from a tremulane framework. Compounds 3 - 7 and 9 - 11 are tremulane sesquiterpenoids of which 4 and 5 are the first tremulane examples with a 1,2-epoxy moiety to be reported. Compounds 6 , 7 , 10 , and 11 possess weak activities to several human cancer cell lines. Compound 8 shows a weak inhibitory effect on NO production with a half maximal inhibitory concentration (IC 50 ) value of 22.7 μM. Compound 1 inhibits the lipopolysaccharide (LPS)-induced proliferation of B lymphocyte cells with an IC 50 value of 22.4 μM, while 2 inhibits concanavalin A (Con A)-induced T cell proliferation and LPS-induced B lymphocyte cell proliferation with IC 50 values of 16.7 and 13.6 μM, respectively.