SUMMARY In immunocompetent rats and humans infection with Toxoplasma gondii remains mostly without overt clinical symptoms, but can be fatal, if the T‐cell response is impaired. For a better understanding of the lack of control of T. gondii infection under immunosuppressed conditions , congenitally athymic rats were used as the experimental model. Whereas athymic F344‐ Whn rnu (F344 nude) rats die from a generalized infection during the first 3 weeks after peritoneal inoculation with 10 6 tachyzoites of T. gondii strain NTE, LEW‐ Whn rnu (LEW nude) rats and euthymic LEW rats infected with a 10‐fold higher number of parasites developed chronic infection. To identify underlying mechanisms of LEW rats resistance to T. gondii infection and to investigate a possible contribution of residual T‐cells to LEW‐ Whn rnu rat resistance, we characterized the immune response of LEW rats by determination of cellularity and composition of lymphocyte population, antigen‐specific IgG2b response as well as assays of antigen‐specific proliferation and production of IL‐2, IFN‐γ and TNF‐α. As only euthymic LEW rats developed production of antigen‐specific IgG and cellular in vitro responses, these results strongly suggest that the genetic background of LEW rats permits a control of the infection independent of an adaptive immune response.