An immunoglobulin-M immunosorbent agglutination assay (ISAGA) was introduced to detect toxoplasma specific IgM. This assay incorporates mu chain capture and use of entire toxoplasma trophozoites as an antigen source. The performance of the ISAGA was compared with that of a double sandwich enzyme linked immunosorbent assay (DS-ELISA) currently used in the Public Health Laboratory Service Toxoplasma Reference Laboratories. The ISAGA was found to be more sensitive than DS-ELISA but there was no demonstrable difference in the specificity or reproducibility between the two assays. The ISAGA is suitable for the diagnosis of acute toxoplasmosis in immunocompetent patients and as a screening test for recent infection in pregnant women. The persistence of ISAGA reactivity, however, is such that additional serological assessment is required to define the risk of congenital infection.