FR-900520 and FR-900523, novel neutral macrolide immunosuppressants, were isolated from the cultured broth of Streptomyces hygroscopicus subsp. yakushimaensis No. 7238. Their molecular formulae were determined as C43H69NO12 and C42H67NO12, respectively. The compounds suppressed immune response in vitro. IC50 values of FR-900520 and FR-900523 for mouse mixed lymphocyte reaction were 0.55 nM and 1.6 nM, respectively. FR-900520, the major component, clearly prolonged skin allograft survival in rats. The detection of immunosuppressants which have the ability to prevent rejection of transplanted organs has been of major interest in our laboratories. A mixed lymphocyte reaction (MLR) has been thought to be the in vitro correlative model for allograft rejection and a representative reaction of interleukin-2 (IL-2) dependent T cell proliferation. On the basis of this, we have searched for efficient immunosuppressants with specific suppression of MLR in cultured broths of microorganisms isolated from soil samples. Previously we reported the discovery of FK-506 which has been proven to be potentially more potent as an antirejection drug than cyclosporine (CsA), the most widely used immunosuppressant at this time. We isolated new compounds FR-900520 and FR-900523 (Fig. 1), both structurally related to FK-506, from cultured broth of Streptomyces hygroscopicus subsp. yakushimaensis No. 7238. The present paper describes the fermentation of strain No. 7238 and the isolation, physicochemical and biological properties of FR-900520 and FR-900523. The determination of their chemical structures will be reported in a later paper.