A comparison of the indirect haemagglutination test (IHAT) with the dye test (DT) in 23 infants with congenital toxoplasmosis showed that in the early part of the first year of life the IHAT titres were nearly all lower than the DT titres. In six cases the IHAT was negative or the titre was so low that the cases could have been missed in routine screening. Later in the first year the IHAT titres became higher than the DT and remained so. The infants belonging to mothers with a high IHAT and therefore assumed to have become infected early in pregnancy had severe infections which included all the cases of hydrocephalus. Infants of mothers with low IHAT titres relative to the DT and therefore thought to have been infected later in pregnancy had generalized, mild or subclinical infection. With some difficulty a suitable commercial conjugated anti-IgM serum was found that gave positive results in immunofluorescent antibody tests for specific IgM antibody with all the infected infants and their mothers throughout the first year and negative results in uninfected infants and their mothers.