Inhibitory activities of A-72363 A-1, A-2, and C, the diastereomers of a neuraminidase inhibitor siastain B, against various glycosidases were tested in comparison to siastatin B. Despite these compounds differing only in their configuration, each compound showed strikingly different specificities towards the various glycosidases tested. A-72363 C inhibited bovine liver beta-glucuronidase and tumor cell heparanase with IC50 values of 1.6 mu M and 12 mu M, respectively.