During the first prenatal serodiagnosis of toxoplasmosis, the test must permit to differentiate between immunized and non-immunized patients and to screen recently contracted toxoplasmosis. In a group of 33 women affected with toxoplasmosis during pregnancy a critical study of serodiagnosis criteria is carried out by comparing the theoretical protocol of the evolution of the serology during acquired toxoplasmosis with the situations observed under usual prenatal monitoring. Seroconversion was noted in 26 women and the variability of the results emphasizes the difficulties in determining the date of the contamination when an evolutive form of toxoplasmosis is suspected at the first examination, which is the case in 7 other patients. In children, 11 congenital toxoplasmosis were diagnosed, all on laboratory examinations. It must be emphasized that 16 children were prematurely lost to follow-up. It should be necessary to devote our energies to screening and information in order to validate the protocol of prevention of congenital toxoplasmosis.