Four marine actinobacteria tolerant to 200 g NaCl l(-1) were screened for antibacterial activity against eight patient-derived multiple drug resistant (MDR) bacteria. The active compound (MW 300.2, predicted molecular formula C20H28O2) from an actinobacterium, was inhibitory to three Gram-positive and three Gram-negative MDR bacteria, seven non-clinical Gram-positive, four Gram-negative bacteria and five fungi (MIC: 3.5-4.0 mu g ml(-1)). Also, 54% of human leukemia (HL-60) cells were killed by the compound at 0.05 mu g ml(-1). Bioreactor production demonstrated unusual primary metabolite kinetics. Molecular phylogenetic analysis showed this typical intertidal inhabitant to be a member of the Streptomyces genus and distinct from other salt-tolerant actinobacteria. As no compound was found to match the properties in several electronic databases, our screening strategy should increase the possibility of discovering bioactive molecules from rare actinobacteria.