Azathioprine (Aza) is an antimetabolite drug, could be labeled with the Auger emitters todine-125 Aza could be used as an ideal vehicle to deliver radioactive decay energy to DNA of tumor cells causing DNA double strand break, thus stop DNA synthesis. In this study, the process of labeling was done via direct labeling technique using chloramine -T as an oxidizing agent and heating to  75 °C, for 30 minutes at pH 7 using 0.5 M phosphate buffer .The radiochemical purity of the labeled compound at the above Conditions, was determined using electrophoresis technique and was above 90% About 2.5 x 10⁶ of Ehrlich Ascites Carcinoma (EAC ) was Injected intrapritoneally (i.p)   to produce stiles and intramuscularly (i.m) in the right thigh to produce solid tumor in  female mice. Biodistribution studies were carried out by injecting solution of ¹²⁵I-Aza in normal and tumor bearing mice The uptake in ascites was over 40 % of the injected dose at 12h post injection and above 20% in solid tumor. The data revealed localization of the tracer in the tumor tissues with high percentage sufficient to give radiotherapeutic effect as well as promising tool for diagnosis.