K04-0144 substances A, B and C which can be produced by culturing a microorganism exemplified by Streptomyces sp. strain K04-0144 (which is capable of producing K04-0144 substances) in a culture medium and then collecting the substances from the culture medium. The substances have a potent antibacterial activity against gram-positive bacteria including methicillin-resistant Staphylococcus aureus (MRSA) and are therefore useful as therapeutic agents for MRSA infection and infectious diseases caused by multidrug-resistant bacteria including β-lactam antibiotic-resistant bacteria. A novel K04-0144 substance D which is also collected from the culture medium can enhance the effect of a β-lactam antibiotic (which has been used as an antibacterial agent) by using it in combination with the β-lactam antibiotic, and is therefore useful as a therapeutic agent for methicillin-resistant Staphylococcus aureus (MRSA) infection and infectious diseases caused by multidrug-resistant bacteria including β-lactam antibiotic-resistant bacteria.