This clinical study was based on the experimental results reported in the two preceding papers, showing that the highly selective affinity of the 123I-anti-CEA monoclonal antibody 47 (123I-Mabgc) for human granulocytes makes this compound suitable for the immunoscintigraphic detection of inflammatory lesions. Forty five patients with suspected infections have been studied after infusion of 4 mCi (148 MBq) 123I-Mabgc corresponding to 120 micrograms labeled protein. No adverse reactions have been seen. Because of the high number of labeled cells, the quality of the images was excellent. SPECT was performed in 15 cases in order to define the extent of the lesion. Infectious foci were usually seen 3-5 h postinjection, but the unimpaired function of the granulocytes guarantees diagnostically relevant examinations over a much longer period of time. Scans were read as being negative if no pathological accumulation of activity was detected after 24 h. The new scanning method is technically easy to perform and provides distinct advantages over other techniques necessitating in vitro labeling of the white blood cells. Therefore, recommended indications are acute infections of unknown origin or extent, especially recurrent episodes of osteomyelitis and infections of joint prostheses.