Since favorable images of infection are obtained with radio-labeled nonspecific IgG, streptavidin has been considered as an alternative protein in this investigation. The advantage of streptavidin is that once localized it may be targeted with radiolabeled biotin. Studies were conducted in a mouse model with an Escherichia coli infection in one thigh. Indium-111-labeled streptavidin showed equivalent localization to the infection as that obtained with 111In-labeled polyclonal nonspecific IgG, however blood levels with streptavidin were lower at all time points; consequently, target-to-blood ratios were improved. Pretargeting with unlabeled streptavidin followed 3 hr later with 111In-labeled biotin showed equivalent localization in the target and reduced activity in all organs sampled. As such, infected thigh-to-normal thigh ratios were improved 3-fold for pretargeting versus either labeled IgG or streptavidin. Improvements in infected thigh-to-liver and blood ratios were greater than 8-fold. Only in the case of kidneys was the ratio unimproved. In conclusion, we have shown that by preadministration of unlabeled streptavidin followed by labeled biotin, infectious lesions in a mouse model may be imaged earlier with lower background levels relative to the administration of labeled nonspecific IgG.