The effectiveness of IgG enzyme-linked immunosorbent assays as serological screening tests for Toxoplasma gondii infection was investigated in three Scottish hospital laboratories. Three assays--two commercial and one produced by the Scottish Toxoplasma Reference Laboratory--were investigated. There was little difference in the sensitivity of the three tests (86-91%), but one of the commercial tests was markedly less specific. The performance of the more specific assays was improved by adjustment to an eight international unit per ml threshold.