The structural and antibacterial activity relationship of 6-alk(en)ylsalicylic acids, also known as anacardic acids, was investigated against Gram-positive bacteria, emphasizing the methicillin resistant Staphylococcus aureus ATCC 33591 (MRSA) strain. The unsaturation in the alkyl side chain is not essential in eliciting activity but is associated with increasing the activity. The antibacterial activity of methicillin against MRSA strains was significantly enhanced in combination with C(12:0)-anacardic acid, and the fractional inhibitory concentration index for this combination was calculated as 0.281. It appears that biophysical disruption of the membrane (surfactant property) is due to the primary response to their antibacterial activity, while biochemical mechanisms are little involved. The compounds possessing the similar log P values exhibit similar activity.