Phoslactomycin B (PLM-B), a potent and selective inhibitor of serine threonine phosphatase is of interest for its antitumor, antifungal and antiviral activity. Described herein is an evaluation of the solution stability of phoslactomycin B at various pH and temperature conditions. Phoslactomycin B was produced from a NP1 mutant strain of Streptomyces sp. HK-803 and purified by semi-preparative HPLC. A study of PLM-B degradation was carried out in the pH range of 210 at 30°C and 50°C using an HPLC assay. The PLM-B decomposition was observed to exhibit a U-shaped pH profile and demonstrated both acid and base-catalyzed decomposition. The decomposition could be described by the equation kOBS=kH10-pH+kOH10pH-14 (kH=457 M-1h-1; kOH=44873 M-1h-1). PLM-B was found to be most stable at pH 6.63. The major acid and base products were separated and purified. Mass spectroscopic and NMR analysis revealed hydrolysis of the ,-unsaturated lactone provided the major degradation product under base conditions. Two other products in which hydration of the ,-unsaturated double bond preceded hydrolysis or methanolysis of the lactone were obtained. Under acidic condition MS and NMR analysis revealed that a dehydration step provided a C9-C11phosphorinane derivative of PLM-B as one of the major products. The remaining acid degradation products were shown to be mixture of various dehydration products containing an additional double bond in central core of the PLM-B carbon skeleton. The major acid and base degradation products had dramatically reduced antifungal activity despite retaining the same structural core.