Somewhat surprisingly, we found that the stability and biological (cytostatic) activity of gemcitabine in mycoplasma-infected tumor cell cultures could be restored by (i) the co-administration of natural pyrimidine (i.e. dThd and Urd) and purine (i.e. Ado, Ino and Guo) nucleosides and (ii) administration of a specific PNP inhibitor. Here, PNP is linked to neoplasm.