Siastatins A and B were isolated as part of a program designed to find Streptomyces-produced inhibitors of sialidase from Clostridium perfringens. Siastatin A was more effective than was siastatin B in the inhibition of sialidases prepared from Cl. perfringens and chicken chorioallantoic membrane. However, siastatin B was a stronger inhibitor of sialidases prepared from streptomyces and rat organs than was siastatin A. Siastatin B also inhibited β-glucuronidase and N-acetyl-β-D-glucosaminidase, two enzymes unaffected by siastatin A. As sialidases from different sources differ in specificity to inhibitors and specific inhibitors might be useful in analyzing the physiological role of this enzyme, we searched for new sialidase inhibitors in culture filtrates of microorganisms. In this communication we describe the isolation, purification and characterization of the inhibitors, siastatins A and B.