9-Propionylmaridomycin shows in vitro antibacterial activity against gram-positive bacteria and has some action on Neisseria gonorrhoeae and Vibrio cholerae, but is generally inactive against many gram-negative rods. This antibiotic exhibits strong activity against clinical isolates of Staphylococcus aureus which are highly resistant to erythromycin or oleandomycin, or both, but which are sensitive to josamycin and kitasamycin. Strains resistant to josamycin and kitasamycin were also found to be resistant to this antibiotic. A significant feature of 9-propionylmaridomycin is a lack in its ability to induce resistance induction in resistance-inducible strains of staphylococci to erythromycin. Several antibacterial features of 9-propionylmaridomycin such as the influence of medium pH, inoculum size, effect of addition of horse serum, development of resistance, cross resistance, and bacteriostatic activity were shown to be almost identical to those of josamycin and kitasamycin. The antibacterial activity of 9-propionylmaridomycin was stable in solutions at pH levels of 4, 7, and 9.