The diagnostic performance of single-serum assays for toxoplasma-specific immunoglobulin (Ig)M. IgA. IgG, and IgE antibodies and of different combinations of such antibody assays in 20 European reference centers was assessed. A panel of 276 sera, of which 73 came from patients who seroconverted within 3 months (acute infection), 49 from patients who had seroconverted 3-12 months earlier (convalescence), and 154 from subjects who had two IgG-positive samples obtained 12 months apart (past infection), was tested with 20 toxoplasma-antibody assays and 195 combinations. In general, every assay with high diagnostic sensitivity showed low diagnostic specificity, i.e. no assay performed alone could reliably distinguish acute from past infection. Furthermore, no single assay (or combination) could separate convalescence from the other stages of toxoplasma infection. However, excellent diagnostic performances were reached by sequential use of highly sensitive IgM assays and methods examining IgG avidity or stage specificity. IgA or IgM assays were less suitable for confirmation of toxoplasma-IgM positivity. This study documents the strength of test combinations in assessing the stage of toxoplasma infection.