Several physicochemical characteristics of the repeated epitope of the major surface protein (P30) of Toxoplasma gondii were investigated with an anti-P30 mAb by two different methods: a one-site/inhibition assay that detects molecules containing single or multiple epitopes and a two-site/one antibody radiometric assay that is only effective with antigenic molecules containing two or more identical epitopes. Using both techniques, the repeated epitope within purified P30 was stable after 1 h at 63 degrees C, but labile at 100 degrees C. It was also resistant to successive freezing and thawing, and not affected after one year at -70 degrees C. Lyophilization and acidic or basic treatment had no effect. This epitope was also resistant to 20% trichloroacetic acid precipitation (activity recovered in the pellet) and to precipitation with cold acetone. To investigate the immunodominance of this repeated epitope during the humoral immune response against T. gondii, competition binding assays between anti-P30 mAb and polyclonal antibodies, from rabbits immunized with either purified P30 or total Toxoplasma extract and from patients with toxoplasmosis, have been used. We found that the mAb inhibited 50-95% of the binding of the IgG antibodies from both rabbits to purified P30. In addition, the binding of both human IgG and IgM antibodies to P30 was significantly inhibited by the mAb. It appears, therefore, that a single region of P30 contains most of the immunogenic activity.