The new substances MR-387A and B (I and II, resp.) were obtained during screening for new aminopeptidase N inhibitors from the culture broth of S. neyagawaensis SL-387.The isolation, physico-chem. properties, structure elucidation, and biol. activities of the inhibitors are reported.Chromatog. methods led to the isolation of 3.6 mg I and 2.0 mg II (as amorphous white powders) from 10 L of culture filtrate.Their structures were established by the use of UV, IR, and NMR spectroscopies and amino acid anal.I and II inhibited aminopeptidase N from porcine kidney microsomes and human fibrosarcoma HT1080 and myelogenous leukemic K562 cell lines.Aminopeptidase B of human myelogenous leukemia K562 cell line was inhibited by I and II with IC50 values in the nanomolar range.The inhibitors had no antimicrobial activity at 100 μg/mg.