In order to characterize Toxoplasma gondii antigens, we have produced a panel of monoclonal antibodies specific for the parasite. A total of 22 hybridomas were derived from the spleen cells of mice immunized either with a 100,000 g supernatant of a sonicate from the RH strain (called F3), or chronically infected with the Wiktor or the 76K strain. Except for one hybridoma producing an IgM, all the hybridomas derived from mice immunized with F3 produced IgG1 antibodies while those obtained from chronically infected mice produced antibodies belonging to the IgG2b, IgG2a and IgM subclasses. Western-blot analysis showed that the panel of monoclonal antibodies defines at least 7 distinct antigens or antigen families. An antigen of apparent Mw 25 kD present exclusively in the 100,000 g supernatant of the T. gondii sonicate was recognized by the majority of monoclonal antibodies derived from mice immunized with the F3 fraction. Two other antigens of apparent Mw 27 kD and 29 kD present in the soluble and insoluble fractions of the sonicate were also identified. Monoclonal antibodies against the previously described 21 kD and 31 kD surface antigens and belonging to the IgG2a but also to the IgG1 subclasses were able to mediate lysis of the parasite in the presence of human non immune serum. The 22 monoclonal antibodies did not identify antigenic differences between the two independently isolated RH and Wiktor strains.