Purified field bean protease inhibitor (FBPI) was labeled with 99mTcO4- to ascertain its ability to locate tumors in tumor-bearing rat models. The labeling was done with Sn2+ as a reducing agent and the yield was 95%. It was stable for 2 hr at ambient temperature. The biodistribution study of the intravenously injected radiolabeled FBPI in normal Wistar rats at various time intervals showed a rapid blood clearance from the systemic circulation (approximately 5hr). The complex was predominantly excreted through the renal and the hepatobiliary systems. In vivo distribution and scintiimaging of 99mTc-FBPI were carried out in rats bearing carcinogen-induced mammary tumor or transplanted C6-gliomas. The results obtained were compared with conventional tumor-seeking radiopharmaceuticals such as 99mTc-(V)dimercaptosuccinic acid (DMSA), 201Thallous chloride (TICI) and 99mTc-Citrate. The tumor to muscle (T/M) ratios obtained with 99mTc-FBPI in rat C6 glioma was nearly 2 to 5-fold higher than obtained with all the three conventional tumor-seeking agents. The T/M ratio obtained with 99mTc-FBPI in rat mammary tumor on the other hand appeared to be 2-3-fold higher than noted with 99mTc(V)-DMSA and 201TlCl. The ratio was however comparable with that obtained with 99mTc-Citrate. The study indicated that 99mTc-FBPI has the specific potentials for imaging gliomas and possibly other tumors as well.