[en] Infectious diseases remain a major health problem and cause of death worldwide, particularly in developing countries. Nuclear medicine imaging, because of its sensitivity, offers an attractive option for diagnosis of focal infections. This needs a reliable radiopharmaceutical that can selectively concentrate in sites of infection. Over the years ⁶⁷Ga and other radiopharmaceuticals that localize in inflammation associated with infection sites, also known as 'non-specific agents' have been used for infection imaging. However, experience has shown that an 'infection specific agent' that concentrates selectively at sites of infection and not inflammation would have several advantages. The first such agent developed more than two decades ago was ¹¹¹In-leucocytes which is still considered a 'gold standard'. Considerations of cost, availability, and superior properties for imaging make ⁹⁹mTc a better label than ¹¹¹In. ⁹⁹mTc white blood cell (WBC) was developed subsequently and used for infection imaging. However, both ¹¹¹In and ⁹⁹mTc WBCs have a number of drawbacks, in particular: each patient's blood sample has to be collected and individually radiolabelled; well-trained staff and suitable facilities for separating and labelling the patient's blood are needed; the risk of infection and cross-contamination associated with potential blood-borne microorganisms; and cost of materials. Because of these, considerable efforts have been continuously made towards developing convenient replacements for ⁹⁹mTc WBCs with limited success, ⁹⁹mTc antigranulocyte antibody being a good example. However, these radiopharmaceuticals still have many disadvantages, related to either their cost and availability or their performance. In view of the large potential for applications in patients, the development of new and improved ⁹⁹mTc labelled infection specific imaging agents was considered as a very worthwhile aim for scientific research in general and, in particular, for the establishment of a Co-ordinated Research Project (CRP) by the IAEA. The CRP could investigate alternate biochemical pathways, promising recent advances in ⁹⁹mTc labelling methodologies and recent progress in evaluation methods. Based on recommendations of two consultants meetings, the IAEA initiated a CRP entitled Development of Kits for ⁹⁹mTc Radiopharmaceuticals for Infection Imaging in 2000. Twelve laboratories from Asia, Europe, North America, and South America participated in the CRP, which was concluded in 2003. Among the objectives of this CRP was the development of different ⁹⁹mTc labelling strategies in participating laboratories that would be useful in the development of ⁹⁹mTc labelled infection imaging agents. In addition, techniques were to be developed for the in vitro and in vivo testing of label stability. Finally, it was hoped that one or more of the identified agents would prove to localize in infection by a specific mechanism. The CRP may be said to be successful in all three measures. Finally, with the identification of ⁹⁹mTc ubiquicidine fragment (UBI 29-41) as a radiolabelled agent with potential clinical utility, this CRP can be considered to have made a major contribution by providing the first validated specific ⁹⁹mTc labelled infection imaging agent