Purpose.A clinico-histopathological cross correlation was made to study the mechanism of tissue damage in toxoplasmic retino-choroiditis during an experimental reactivation of chronic toxoplasmosis and to compare the influence of treatment by sulfaddiazine on the retinal lesions.Methods.Chronically infected Swiss-Webster mice were treated, six weeks after infection, with an avirulent strain ofToxoplasma gondii(Beverley strain) with polyclonal rabbit antibody directed against murine interferon gamma.Results.Mice treated by anti-interferon gamma developed clinical lesions between day 15 and day 30 (lesions including single foci of retinochoroiditis, multifocal lesions or diffuse areas of retinal necrosis). These lesions did not arise from borders of pre-existing scars. The retina was photographed with an operating microscope fitted with a 90 diopter lens. Biological study showed a significant rise of parasitic loads in the eye and brain. Histological examination is in favour of free organism dissemination via retinal vessels; the lesions are restricted to the inner retina and ciliary body, the parasites migrated from extra-ocular cysts via the vasculature. No cysts were seen at the beginning of the study; they were found at the scar phase and appeared in mice treated with sulfadiazine. The clinical lesions were not caused by cysts but by coagulated necrosis in the retinal tissue. Parasite migration may have played a trigger role.Conclusions.The retinal damage was constituted either as a result of a toxic effect of the organisms or as a hypersensitive reaction to the toxoplasma organism. The results of this study showed that the treatment with anti interferon gamma was sufficient to reactivate chronic infection.