Radiolabeling procedures may modify the structure of the Fc portion of the immunoglobulin molecule in such a way that its in vivo immunological behavior may be altered and its efficacy as radiopharmaceutical for inflammatory lesions impaired. This study tested the efficacy of thiol reduction-mediated 99mTc human IgG for scintigraphy of focal inflammatory lesions, either bacterially or chemically induced and located either in the abdominal/thoracical region or in the thigh of baboons. Positive images were obtained in the thigh lesions between 4 and 7 hr after i.v. administration of the labeled IgG. The abdominal/thoracic lesions were never very clear, mostly because of very hot kidneys. Late visualization (20 hr) of all lesions was poor and a high background was present. Bacterially induced lesions were better visible, although no neutrophil nor monocyte activity could be established in the mechanism of the IgG localization.