Immunoscintigraphy with radiolabelled monoclonal antibodies has been widely used to detect solid tumours. The purpose of this study was to investigate its potential for the specific localization of syphilitic lesions. F(ab')2 fragments were prepared from murine monoclonal antibodies against Treponema pallidum produced in our laboratory and labelled with 131I. Bilateral testicular infections were created in rabbits by inoculation with T. pallidum or Staphylococcus aureus. Seven to 10 days after inoculation, radiolabelled antibodies were injected intravenously. Serial gamma images were then taken 2 h after the injection and at 24 h intervals thereafter. Beginning as early as 2 h post-injection, the testicles could be visualized with either specific or non-specific antibodies. Gamma images in the monoclonal antibody-treated, T. pallidum-infected group persisted up to 48 h post-injection. Lesions were not discernible from background in the S. aureus-infected group injected with the monoclonal antibody and the S. aureus-infected and T. pallidum-infected groups injected with the polyclonal antibody at 24 h post-injection or later. Therefore, due to its ability to differentiate between specific and non-specific antibody-generated images from 24 h post-injection, immunoscintigraphy using monoclonal antibodies specific for T. pallidum may be employed as one of the methods to diagnose difficult cases of syphilitic internal organ involvement as well as syphilis infection in seronegative HIV-infected patients.