Interleukin 6 (IL-6) is a multifunctional cytokine that acts on immune response, hematopoiesis, acute phase reactions and the nervous system1*. Thus, IL-6 plays one of the central regulatory roles in host defense mechanisms. However, it has been demonstrated that IL-6 produced by tumor cells causes cancer cachexia, and also stimulates the proliferation of tumor cells in an autocrine/paracrine manner2~5). Therefore, it may be possible that the inhibition ofIL-6 activity relieves cancer cachexia and suppress the growth of IL-6 dependent tumor cells.In the course of a search for inhibitor of IL-6 activity of microbial origin, we have reported a new IL-6 inhibitor, chlovalicin, together with ovalicin produced by Sporothrix sp. FO-46496'7). Continuation of the screening, led to the discovery of new selective inhibitors of IL-6 activity, named madindolines A (1) and B (2) in the cultures of Streptomyces sp. K93-071 1 isolated from a soil sample (Fig. 1). Taxonomic studies of the procharacteristics of 1 and 2 were reported in a previous paper8). This paper describes physico-chemical properties and structural determination of 1 and 2.