In order to evaluate antibacterial activities of combination uses of isepamicin (ISP) and beta-lactams in vitro, minimum inhibitory concentrations (MICs) these drugs were examined singly and in combination against clinically isolated Staphylococcus aureus. The results are summarized as follows; 1. MICs of ISP + cefazolin (CEZ), ISP + cefotiam (CTM) and ISP + flomoxef (FMOX) were low and the activities against methicillin (DMPPC)-susceptible S. aureus (MSSA) were dependent on the concentration of ISP. Combined effects were observed when the concentrations of ISP were at sub-MIC levels (1/2 approximately 1/4 concentrations). 2. MICs of ISP + CEX, ISP + CTM, ISP + FMOX, ISP + imipenem and ISP + panipenem were low and the activities against DMPPC-resistant S. aureus (MRSA) were dependent on the concentration of ISP, and were similar to those against MSSA. Combined effects were observed when the concentrations of ISP were at sub-MIC levels of ISP. Lower MIC50 or MIC90 was observed at ISP concentrations of 4 approximately 16 micrograms/ml. 3. The blood Cmax of ISP exceeded 20 micrograms/ml at one-time administration of ISP 400 mg, and these results suggested that antibacterial activities of combination uses of ISP and beta-lactams was clinically effective against MRSA infections.