The aim of this study was to characterize the antibiotic activity associated with a cell-free extract of marine bacterium Pseudoalteromonas sp. SW21. This bacterium produces a potent antibiotic compound that inhibited the growth of sixty-one strains of bacteria and delayed the growth of ten strains of bacteria and fungi. The compound was produced, in its most robust form, in 3-day-old bacterial lawns grown on agar, and was stable at both 4℗ʻC and -20℗ʻC. Physical and chemical analyses suggest that the compound is a 75 - 100-kDa protein. These characteristics, along with the significance of the environment that Pseudoalteromonas sp. SW21 was isolated from, indicate that the antibiotic activity exhibited by the SW21 cell-free extract may be due to a new class of antibiotic with a novel mechanism of action, making the compound ideal for further drug development.