Four different strains of Toxoplasma gondii were used as antigens indirect immunofluorescence reaction (RIFI) in order to study the influence of the titers of the antibodies. One of the strains ("C" strain) was isolated from a human congenital toxoplasmosis case and the other three ("I", "S" and "E" strains), from human lymphoglandular toxoplasmosis cases. Of the 72 human sera examined by RIFI, 13 were found to be non-reagents in the 1:16 dilution when using each of the 4 strains as antigen. In only 5 sera there was agreement of the antibodies titers when using the 4 strains as antigen, one of them being concordant in the 1:16 dilution and the other four in the 1:64 dilution. In the remaining 54 sera examined, different titers were observed in one to three, four foulds dilutions, according to the strain used as antigen. Statistical testing of results showed these differences to be significant in relation to the "C" strain and non-significant at a 5% level in relation to the "I", "S" and "E" strains. The tendency to more elevated titers of serum antibodies when using the "C" strain may be related to antigenic variations. For a good reproductiveness of results by the RIFI it is necessary to standardize not only the reaction techniques, optical equipament and reagents, but also the antigen used.