During the course of T. gondii infection, we have analysed serum IgG and IgA antibodies responses in 50 immunocompetent with acquired infection and 19 immunocompromised patients with evidence of reactivated toxoplasmosis. Using an ELISA, IgG1, IgG2, IgG3 and IgA antibodies were found in sera of all patients, whereas IgG4 antibodies were usually not detectable. In immunocompetent patients, the predominant antibody isotype was IgG1 at the different stages of infection, presumably in relation with a T-cell control of humoral response during toxoplasmosis. In immunocompromised hosts (kidney or bone marrow transplanted and HIV infected patients), a sequential study was performed on serum samples taken before and after reactivation had occurred. The isotypic distribution of antibodies was similar to that observed in immunocompetent patients, but differences between groups of immunocompromised patients were detected when the kinetics of the antibody response was considered. The IgG and IgA antibody rise was lower in HIV1 infected patients with clinical toxoplasmosis; whatever was the peak antibody value, clinical symptoms appeared earlier in patients with a slower antibody response. This presumably reveals a functional T-cell abnormality, which may rely to the defective containment of the parasite in these patients.