Serine/threonine phosphatase type 2A (PP2A) is an intracellular protein phosphatase, which catalyzes dephosphorylation of many substrates. We have recently found that specific inhibitors of PP2A augment natural killer cells in vivo and inhibit tumor metastasis1-3). Thus, a specific inhibitor of PP2A is a candidate for a new immune activator. In the course of our searching for a novel PP2A inhibitor, we have found that the culture broth of Fungus strain F-1531 showed potent inhibitory activity against PP2A. We isolated active materials including two new compounds. These compounds were found to be active only under the chelated condition with zinc ion. In this paper, we describe the fermentation, isolation, physico-chemical properties, and biological activities of harzianic acid-related compounds.