[en] The search of specific radiopharmaceuticals to be used in breast tumour diagnosis is relevant to complement the techniques applied in conventional medicine. Crotalus durissus terrificus venom (CV) and its main polypeptide, Crotoxin (Crtx), are natural source of several bioactive substances with therapeutical potential. The aim of this work was to evaluate the binding of Crtx with tumour targets in vivo, as well as, evaluate its applicability for breast tumours diagnosis. Crtx was labelled with ^125/131I using lactoperoxidase method and radiochemical analysis was performed by chromatography. ¹²⁵I-Crtx was used for biodistribution and pharmacokinetics studies on swiss mice bearing Ehrlich solid tumour, while ¹³¹I-Crtx was used for single photon emission computed tomography (SPECT) imaging. Crtx presented specific binding sites on Ehrlich tumour cells and had a rapid blood clearance (T_1/2= 201.1 min.). Intratumoral administration increased significantly the activity delivered into the tumour site (128-fold higher) and reduced the kidney burden (7.2-fold lower). ¹³¹I-Crxt demonstrated to interact with tumour cells for until 72 hours allowing good quality images of tumour. Our results indicate the biotechnological potential of Crtx as template for radiopharmaceutical design for cancer diagnosis. (author)