In the present study, radioiodinated human recombinant interleukin-1 (IL-1) was investigated for its potential to image infectious foci in vivo in an animal model of infection. Twenty-four hours after induction of a Staphylococcus aureus abscess in the left calf muscle, mice were i.v. injected with both iodine-125 labelled IL-1 and iodine-131 labelled myoglobin, a size-matched control agent. The animals were killed for tissue biodistribution studies at 2, 6, 12, 24 and 48 h p.i. Gamma camera images were obtained at 6, 24 and 48 h after injecting mice with 123I-IL-1. Radioiodinated IL-1 rapidly cleared from the body; after 12 h the abscess was the organ with the highest activity. The absolute abscess uptake of 125I-IL-1 remained high compared to 131I-myoglobin, resulting in significantly higher abscess-to-muscle ratios of 125I-IL-1 compared to 131I-myoglobin. The ratios of 125I-IL-1 reached the ultimate value of 44.4+/-10.8 at 48 h p.i., whereas the ratios of 131I-myoglobin did not exceed 5.9+/-0.7. Gamma camera imaging revealed clearly visible abscesses. In conclusion, our results demonstrate specific retention of radioiodinated IL-1 in the abscess, presumably by interaction of IL-1 with its receptor on the inflammatory cells. The high target-to-background ratios that were obtained over the course of time indicate that the IL-1 receptor may be a valuable target for the imaging of infectious foci.