One subfraction from the hexane fraction of Acri graminei Rhizoma, the E4 fraction which is mainly consisted of acorenone, showed a potential inhibitory activity against chloramphenicol acetyltransferase (CAT) of S. aureus SA2 that is a multidrug-resistant strain to 10 usual antibiotics. The combination therapy of this fraction with chloramphenicol resulted in reduction of the minimal inhibitory concentration from 128 $mu extrm{g}$/ml to 8 $mu extrm{g}$/ml. The E4 fraction also revealed to prevent the induction of CAT from this strain.