RH strain Toxoplasma gondii was used to inoculate mice and rats. Sera collected at intervals were assayed by the indirect fluorescent antibody test (IFAT) and the latex agglutination test (LAT) to monitor the time-course change in titers of anti-toxoplasma IgM and IgG antibodies. In addition, the above sera and some anti-toxoplasma seropositive human sera (with IFAT antibody titers greater than or equal to 1:16) were adsorbed with staphylococcal protein A (SpA) and assayed for changes in IgM and IgG antibody titers so as to evaluate the usefulness of SpA adsorption in detecting anti-toxoplasma IgM which shows up early in toxoplasma infections. Samples assayed included 262 sera from mice, 65 sera from 5 rats and 85 human sera. The results revealed that parasite specific IgM-IFAT antibodies were detectable in 40% of the sera from mice 3 days after infection. After SpA adsorption, however, the IgM-IFAT antibody could be detected in a few specimens as early as 2 days post infection. IgG-IFAT and LAT antibodies first appeared in the sera on the 5th day of infection. In mice inoculated with freeze-killed tachyzoites (immunized mice), IgG-IFAT and LAT antibodies remained high from 16-35 days after inoculation, whereas IgM-IFAT antibodies were undetectable. Even after SpA treatment, only about half of the 16-day samples showed IgM antibody titers and the other specimens still remained negative. In general, IgM antibody titers increased 2-6 fold after SpA adsorption while IgG antibodies were almost completely removed after the treatment with a residual IgG-IFAT titer of less than or equal to 1:4.(ABSTRACT TRUNCATED AT 250 WORDS)