Abstract: Latent Toxoplasma infection in mice inoculated with the low virulent Beverley strain (BEV) and avirulent Fukaya strain (FUK) was reactivated by prednisolone administration: Parasitaemia reappeared and the organism could be continuously isolated from the lymph nodes by mouse passage. No distinct decrease in specific antibody levels was observed when the sera were assayed by dye test and latex agglutination test. A significant increase in the number of brain cysts in infected mice was noted at the third week of infection while the challenge infection and steroid immunosuppression treatment resulted in an enha-ncement of cyst formation. A hypothesis was, therefore, proposed that the cyst fromation in tissues might be regulated by the immune response of infected host to Toxoplasma infection.