We have previously reported that benastatins A, B and C, and bequinostatins A, B, C and D were isolated from the culture broth of Streptomyces sp. MI384-DF12 and are novel inhibitors of glutathione S-transferase (GST, EC 2.5.1.18). In our continuing study of the strain, we have found that this strain produces another inhibitor designated cysfluoretin (1) which does not contain a benzo[a] naphthacene skeleton and differs from benastatins and bequinostatins as shown in Fig. 1. Here we wish to describe the isolation, physico-chemical properties, structure and biological activities of 1.