Members of the class Actinobacteria are known to produce pharmaceutically useful compounds and have been extensively studied. Soil has thus far been the primary source of Actinobacteria. However, the rate of novel compound discovery from these terrestrial strains has significantly decreased recently. Therefore, we wanted to explore the possibility of isolating unusual Actinobacteria capable of producing new metabolites by using clearly devised methods instead of general selection methods using antibiotics. For this purpose, we isolated Actinobacteria from soil samples using the membrane filter (MF) method,1,2 which allows for selection of Actinobacteria without using antibiotics. In fact, we have already reported the isolation of a new species of Streptomyces by the MF method.2 Our attempts to discover novel compounds from these actinobacterial origin resulted in the successful isolation of new promothiocin derivatives designated as JBIR-83 (1) and JBIR-84 (2) from the culture broth of Streptomyces sp. RI19 (Figure 1a). In this paper, we report the fermentation procedure to obtain 1 and 2 and their subsequent isolation, structure elucidation and in brief, their biological activities.