One hundred twenty-nine European laboratories participated in a collaborative, multicentre study designed to evaluate the overall reliability of different serological techniques for diagnosis of Toxoplasma gondii infection. Five freeze-dried reference sera were distributed to each laboratory, each of which analysed the sera with its routine methods. The enzyme-linked immunosorbent assay was the technique used most frequently, followed by the immunofluorescent antibody technique. Only nine laboratories performed the Sabin-Feldman dye test. In general, there was good concordance between qualitative results, but for sera with low concentrations of Toxoplasma gondii-specific IgG antibodies, some false-negative results were found. For specific IgM and IgA antibodies, the immunosorbent agglutination assay proved the most sensitive. The present study demonstrates the need for regular assessment of laboratory serodiagnosis of Toxoplasma gondii infection.