OBJECTIVE The diagnosis of congenital toxoplasmosis includes a postnatal follow-up, often preceded by a prenatal diagnosis. The aim of our study was to assess the performances of the different techniques used and the value of the samples in the postnatal biological diagnosis. METHODS The methods available between 1985 and 1993 consisted in the detection of: i) Toxoplasma gondii in the placenta; ii) anti-T. gondii IgM in infant's blood by enzyme-linked immuno-sorbent assay (ELISA), immuno-sorbent agglutination assay (ISAGA) and indirect immuno-fluorescence (IFI), and anti-T. gondii IgG by ELISA and IFI; and iii) neo-synthetized anti-T. gondii IgG and IgM by enzyme-linked immuno-filtration assay (ELIFA). RESULTS Among 400 cases of seroconversion diagnosed during pregnancy, a sure diagnosis with complete follow-up could be established for 104 infants; 37 of them had proven congenital toxoplasmosis (CT+) while 75 had no congenital toxoplasmosis (CT-). Biological arguments supporting congenital toxoplasmosis had been observed as early as birth in 78.4% of CT+ cases and before two months in 94.6%. The serologic tests were positive in 88.2% of CT+ cases by ELIFA, in 73.0% by ISAGA, in 43.3% by ELISA M and in 14.0% by IFI M. ELIFA was the less specific method (91.3%). CONCLUSIONS The sensitive techniques (ELIFA and ISAGA), were essential for the instant follow-up to detect toxoplasmic infection as early as birth.