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Technetium-99m (99mTc) labelled polyclonal human immunoglobulin (HIG) is a new agent for the localization of active inflammatory diseases. The results obtained with HIG in 29 AIDS patients referred for suspected lung infections are reported (Table I). The patients also underwent Gallium-67 citrate scanning (GS), chest radiography (Rx), high-resolution thin-layer computed tomography (HRCT) and broncho-alveolar lavage (BAL). The study population was classified as follows: 12 patients (Table II) were studied before treatment for suspected Pneumocystis carinii pneumonia (PCP), 7 patients (Table III) had known PCP and were studied during medical therapy, and 10 patients (Table IV) had lung infections other than PCP. In all PCP patients studied before treatment, positive agreement was observed between HIG, Rx and HRCT findings. In 4 patients with final clinical diagnosis of no lung conditions, both nuclear and radiologic imaging were negative. 99mTc-HIG results in the PCP patients studied during therapy were consistent with clinical and radiologic improvement; there was disagreement with 67Ga findings in one case (no. 9). In lung infections other than PCP, HIG studies were often negative (always negative in mycobacteriosis), while they were positive in 3 pyogenic abscesses. In conclusion, as for PCP and abscesses, the results obtained with 99mTc-HIG are usually in agreement with GS findings, while HIG scans seem to be negative in mycobacterial infections. Moreover, HIG scintigraphy seems to be suitable for the evaluation of treatment results in PCP (this subject deserves further research). To assess respiratory impairment a semiquantitative index (ISQ) of 99mTc-HIG lung uptake is suggested, which showed a significant linear correlation with arterial pO2.