We have synthesized derivatives of dephostatin, a protein tyrosine phosphatase (PTPase) inhibitor, to study the structure-activity relationships of this inhibitor. Inactive analogs revealed some insight into structural requirements or PTPase inhibitory activity of dephostatin. Both a nitroso group and phenolic hydroxyl groups were found to be essential for the inhibitory activity. Among the dephostatin derivative synthesized, one of the regioisomers of dephostatin showed PTPase inhibitory activity equivalent to that of dephostatin, and also had increased stability.