The usefulness of IgG avidity analysis for dating acquired toxoplasmosis was documented by a study of 145 serial sera from 39 patients with acute toxoplasmosis and 104 sera from patients with chronic toxoplasmosis. ELISA measurement of IgG avidity involved comparison, for each serum at limit dilution, of optical densities obtained with and without washing with a urea solution to disrupt antigen-antibody bonds. A significant correlation was found between time since onset of toxoplasmosis and IgG avidity. Furthermore, comparison of IgG avidity of sera from patient with chronic infection or with acute infection monitored serologically for less than 20 weeks showed that an avidity index of 0.5 or more was inconsistent with toxoplasmosis of less than 20 weeks duration.