There are still controversial views as to the relation between SLE and toxoplasmosis. Therefore, we looked for serological markers in both diseases. In patients with SLE (17), toxoplasmosis (28), and in normal controls (28) anti-Toxoplasma gondii antibodies, anti-nuclear antibodies of different specificities, anti-histone and anti-cardiolipine antibodies, as well as antibodies against most common public idiotypes were measured. Significant increases in antinuclear antibodies and other SLE-related antigens were observed in patients with SLE. On the contrary, low levels of these antibodies were found in toxoplasmosis patients and in controls. The same was true for 16/6 anti-DNA idiotype antibodies. The incidence of anti-Toxoplasma antibodies in SLE sera did not differ in comparison with that in the normal population. Our data suggest that subacute and chronic toxoplasmosis do not play essential roles in generating the antibodies that are important to the pathogenetic mechanism operating in SLE.