An enzyme linked immunosorbent assay (ELISA) based on the antibody class capture method for the detection of specific IgM against Toxoplasma gondii, using the microtitre plate format, was developed. Antigen binding was detected using a monoclonal antibody, CIE3, conjugated to horseradish peroxidase. Prior mixing of the conjugate and antigen improved the stability of these reagents as well as removing an incubation stage from the assay. The incubation time of less than four hours permits a rapid throughput of specimens. Using the assay, a total of 163 sera were examined in a three centre study and good agreement was found. Results were expressed as arbitrary enzyme immunoassay units (EIUs) against a freeze dried standard. Throughout the study the standard serum showed a coefficient of variation less than 10% across the microtitre plate. By measuring IgM titres in patients having toxoplasmic lymphadenopathy with a known date of onset, IgM class antibodies were shown to peak at two months, persisting for about six months. In addition, a case of laboratory acquired toxoplasmosis was monitored. Sera shown to contain rheumatoid factor and antinuclear factor did not give false positive results. This rapid, robust, and simplified assay is used by the Public Health Laboratory Service Toxoplasma Reference Units and will provide a standard with which other assays can be compared.