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Abstract. Technetium-99m labelled antigranulocyte antibodies are ready to use and are sensitive and specific in the diagnosis of infectious diseases. [sup 99m]Tc-SSEA antigranulocyte antibodies have a very high affinity constant (K[sub d] 10[sup -12] M) for human neutrophils (PMNs), and excellent imaging qualities with high target/background ratios. The aim of this study was to compare the diagnostic accuracy of the [sup 99m]Tc-anti-SSEA-1 monoclonal antibody (Mab) with that of [99m]Tc-hexamethylpropylene amine oxime (HMPAO)-labelled white blood cells (WBCs). To this end, 17 patients with 23 proven infectious foci were examined with 555 MBq [sup 99m]Tc-anti-SSEA-1 MAb and with 370 MBq [sup 99m]Tc-HMPAO labelled autologous leucocytes within a period of 7 days. All the infections were confirmed by culture, biopsy, surgery and follow-up. Whole-body images and planar spot views with the antibody were performed at 1-h, 4-h and 24-h post injection; the biodistribution of the antibody was quantified, absorbed radiation doses were calculated and the diagnostic results were compared with the [sup 99m]Tc-HMPAO WBC images. Human anti-mouse antibody (HAMA) evaluation was performed in all patients before and 3 months after antibody injection. Blood was drawn at different times after [sup 99m]Tc-anti-SSEA-1 MAb injection to determine the amount of granulocyte-associated radioactivity and to calculate recovery. [sup 99m]Tc-anti-SSEA-1 MAb scintigraphy detected all 23 lesions, while 21 were detected with [sup 99m]Tc-HMPAO WBC scan. In this small group of patients, the sensitivity and specificity of [sup 99m]Tc-anti-SSEA-1 MAb scintigraphy were 95% and 96% respectively, as compared with 91% and 82% respectively for [sup 99m]Tc-HMPAO WBC scan. An increasing uptake of the injected activity in the lesion at different time points was indicative of high affinity and of specific PMN binding. There was no HAMA formation. In four of five patients investigated, a transient mild leukopenia w [ABSTRACT FROM AUTHOR]