Telomerase is a ribonucleoprotein enzyme which is involved in the maintenance of chromosome ends. Telomerase activity (TLMA) is required for cell immortality and can be considered a ubiquitous tumor marker. In this study we describe a new approach for developing in vitro chemosensitivity assays based on the assessment of TLMA in tumors, upon treatment with antineoplastic agents. The results of preliminary studies on in vitro cultured tumor cell lines indicate that TLMA inhibition can be used as a marker of tumor cell killing by anticancer drugs. Therefore the present investigation provides the rational basis for an in vitro chemosensitivity assay based on TLMA evaluation.