Antimicrobial activity of GL (the aqueous extract from the carpophores of Ganoderma lucidum (FR)KARST) was tested in vitro against Gram positive and Gram negative bacteria by serial broth dilution method, and the antimicrobial activity was expressed by minimal inhibitory concentration (MIC). Among fifteen species of bacteria tested, the antimicrobial activity of GL was the most potent against Micrococcus luteus (MIC, 0.75 mg/ml). To investigate the effects of antimicrobial combinations of GL with four kinds of antibiotics (ampicillin, cefazolin, oxytetracycline and chloramphenicol), the fractional inhibitory concentration index (FICI) was determined by checkerboard assay for each strain. The antimicrobial combinations of GL with four antibiotics resulted in additive effect in most instances, synergism in two instances, and antagonism in two instances. Synergism was observed when GL was combined with cefazolin against Bacillus subtilis and Klebsiella oxytoca.