Abstract: The present invention provides a method for up-regulating tumor cell antigen expression, comprising administering to the cells an amount of thymalfasin sufficient to increase the expression of TLP relative to that of untreated tumor cells. Also provided are methods for enhancing the sensitivity of a immunodiagnostic or immunotherapeutic method, comprising pre-treating target tumor cells by administering to the cells an amount of thymalfasin sufficient to increase the expression of TLP relative to that of untreated tumor cells, followed by application of the immunodiagnostic or immunotherapeutic method. These methods are applicable to both in vivo and in vitro diagnostic methods, and to in vivo immunotherapeutic methods.