We have developed a method for obtaining cysts of Toxoplasma gondii RH strain. Outbred Swiss Webster mice were infected subcutaneously (s.c.) or intraperitoneally (i.p) with 10(5) tachyzoites and given sulfadiazine 400 mg l-1 + NaHCO3 10 g l-1 in drinking water from day 1 to day 15 post-infection (p.i.). None of the mice infected i.p. survived, compared with 50% of the mice infected s.c. Cysts were detectable in the brain on day 45 p.i., and had ultrastructural features consistent with those of bradyzoites. However, these cysts were incapable of infecting mice via the oral route. In addition, immunofluorescence studies showed the persistence of P36 protein expression, indicating that the conversion to bradyzoites was incomplete.