The diagnosis of acute toxoplasmosis usually depends on serology, yet little data are available to compare the relative usefulness of various serologic tests after the onset of illness. The Sabin-Feldman dye test (DT), the IgM immunofluorescent antibody (IgM-IFA) test, the soluble antigen complement-fixation (CF) test, and the indirect hemagglutination (IHA) test were performed on serial serum specimens from 27 previously healthy patients, each of whom could identify the date of onset of illness within two weeks. IgM-IFA titers of greater than or equal to 1:160 were the best indicators of infection acquired in the past two to four months. The DT was useful for screening, but two-tube rises in titer were rarely documented, and absolute titers were imprecise indicators of the recentness of infection. Although two-tube rises in titer in CF and IHA tests could be seen in the majority of patients, the rises were so slow that both tests were less useful than the IgM-IFA test in documenting the diagnosis of acute toxoplasmosis.