AIMS--To examine the serological response of patients with systemic lupus erythematosus (SLE) and toxoplasma infection and to compare the blot profiles with those from immunocompetent subjects of similar immune response. METHODS--Forty serum samples from patients with SLE were tested for toxoplasma antibodies using the dye and indirect haemagglutination tests. Specific IgM was measured by mu-capture enzyme linked immunosorbent assay (ELISA). The sera were immunoblotted using antigen strips prepared from the RH strain of Toxoplasma gondii. For comparison, control blots were prepared from pooled sera from immunocompetent subjects with serological evidence of acute (pool 1), or chronic (pool 2) toxoplasma infection, or with no evidence of infection (pool 3). RESULTS--Some of the blot profiles from the patients with SLE were compatible with the corresponding serology but others showed considerable variation, particularly among the IgM blots. The blots from sera with low dye test titres suggested that the latter could be false positive results. CONCLUSIONS--Toxoplasma infection may enhance the production of autoantibodies which, when combined with the high titres characteristic of SLE, might interfere in the dye test and other serological tests. Immunoblotting could prove useful in the immunocompromised for confirming the presence of specific toxoplasma antibodies and for the staging of infection in those with positive serology.