In an effort to contribute to the understanding of the mechanism of uptake of technetium-99m labelled non-specific polyclonal human immunoglobulin G (hIgG) in inflammatory lesions we compared the tissue distribution of double-labelled 99mTc-hydrazinonicotinamido (HYNIC) hIgG-14C and 99mTc-iminothiolano hIgG-14C in groups of five Wistar rats with a Staphylococcus aureus infection of the left calf muscle between 2 h p.i. and 24 h p.i. The stability of the two double-labelled hIgG preparations was evaluated in vitro and in plasma in vivo by high-performance liquid chromatography (HPLC) analysis. At 24 h after injection of 99mTc-HYNIC-hIgG-14C the abscess uptake of 99mTc (1.5% ID/g+/-0.2% ID/g) was significantly higher (P<0.01) than the 14C uptake (1.0% ID/g+/-0.1% ID/g). After injection of 99mTc-iminothiolano hIgG-14C no significant difference (P=0.08) was found between the abscess uptake of the two radionuclides at 24 h p.i. (99mTc: 0.8% ID/g+/-0.1% ID/g; 14C: 0.90% ID/g+/-0.09% ID/g). HPLC analysis of plasma samples revealed release of 99mTc from both double-labelled immunoglobulin preparations. This phenomenon was more pronounced for iminothiolano hIgG than for HYNIC hIgG (43% vs 18%). In most tissues other than abscesses significant differences were also found between the 99mTc and the corresponding 14C uptake. Our results demonstrate that the chemical form in which 99mTc is bound to hIgG severely influences its release from hIgG and its retention in infections.