The in vitro activity of coumermycin A(1) was compared with that of novobiocin, ampicillin, and minocycline. Coumermycin was found to be the most active antibiotic of the four against Staphylococcus aureus. It was about 50 times more active than novobiocin or minocycline against the strains tested. Coumermycin also showed good activity against group A streptococci and pneumococci, moderate activity against Escherichia coli, indole-positive Proteus species, and Pseudomonas aeruginosa, and poor activity against Klebsiella-Enterobacter and enterococci. Against P. mirabilis, however, coumermycin activity was almost equal to that of ampicillin. The new antibiotic was further found to be greatly reduced in activity in the presence of plasma, but its minimal inhibitory concentration was not greatly affected by inoculum size. Coumermycin was found to be bacteriostatic in its action, and resistance to it developed slowly. Also, cross-resistance was present with novobiocin but absent with ampicillin or minocycline.