The abilities of99Tcm-labelled monoclonal anti-human granulocyte antibody (AGAb) and111In- labelled nonspecific polyclonal human immunoglobulin (IgG) to localize at focal sites of inflammation were compared in rats with deep thigh infection due to E. coli. The radiolabelled antibodies were coadministered followed 4-6 and 24 h later by imaging and biodistribution studies. At 4-6 h after injection, the target to background ratio (T/B, lesion to contralateral leg) and percentage residual activity (% RA, counts in the lesion/total body counts) were nearly identical for both antibody preparations. At 24 h, T/B and % RA increased significantly (P<0.001) for both proteins but differences between the agents were not significant. In vitro analysis of the binding of AGAb and human polyclonal IgG to rat granulocytes showed a low level of binding with both agents. These results suggest that the primary mechanism of localization, by either antibody preparation in this model, is not antigen related. nlIn-labelled nonspecific human IgG and99Tcm-AGAb are equivalent reagents for the detection of focal sites of infection in the rat.