Abstract Objective: To develop an enzyme-linked immunosorbent assay (ELISA) for measuring Toxoplasma gondii IgG avidity that combines the accuracy of end-point titer assays with the ease of optical density index assays. Methods: Sera collected from 24 patients within 5 months of onset of toxoplasmosis (Recent Infection Group) and sera from 25 patients who had toxoplasmosis many months or years earlier (Past Infection Group) were tested in the prototype T. gondii IgG avidity assay. The avidity index (AI) in this assay was based on differential T. gondii-specific IgG reactivity in serum-treated wells washed with urea buffer versus control buffer; unlike previously-described assays, however, the IgG reactivity was measured quantitatively using a standard curve. A subset of sera was also tested using a commercially available ELISA based on end-point titration, and these results were compared to those obtained using the prototype assay. Results: All sera in the Recent Infection Group exhibited AI values <0.18 in the prototype assay, whereas all sera in the Past Infection Group exhibited AI values >0.27. AI values of the Recent Infection Group were significantly correlated with days post onset of symptoms. The correlation between AI values in the prototype ELISA and AI values in a commercially available assay was highly significant (R 2=0.9125). We also confirmed and extended the findings of other investigators, showing that AI values calculated using optical density values showed significant variation depending on the serum dilution used, whereas AI values calculated using quantitative IgG values did not. Conclusion: A simplified ELISA for T. gondii IgG avidity based on quantitative IgG values correctly discriminated recent from past toxoplasmosis, and showed excellent correlation with a commercially available, but more complicated, assay. This new assay should facilitate the accurate measurement of T. gondii IgG avidity in a reference laboratory setting.