In vitro lymphocyte proliferative response of peripheral blood leucocytes (PBL) to purified Toxoplasma gondii antigen were evaluated by 3[H] methyl thymidine incorporation in patients acutely and chronically infected with Toxoplama gondii. PBL from three patients with acute sylnptonlatic toxoplaslnosis showed no response to T. gondii antigen during the emergence of anti-Toxoplasma 1gM antibodies and the response returned as the infection became chronic. Lymphocytes of twelve chronically-infected patients responded positively to the antigen. In all patients the lymphocyte proliferative response to the mitogen, Concanavalill A (Con A) was normal. Analysis of Toxoplasma proliferative response of PBL from a patient with acute toxoplasrnosis showed that CD8+ cells were responsible for induction of suppression while the response during the chronic infection was lnediated by CD4+ cells. In human toxoplasmosis there was antigen-specific lymphocyte unresponsiveness during the acute phase of the infection and it appears that the initnunesuppression was mediated by CD8+ cells. KEYWORDS: Toxoplasmosis-lymphocyte blastogenesis-antigen specific-CD4+, CD8+