In the preventive treatment of congenital toxoplasmosis it is admitted that spiramycin is concentrated in the placenta and crosses the placental barrier at the end of pregnancy. However, little is known about its entry in the fetal circulation. We studied fetal spiramycin blood levels during the 2nd and 3rd trimesters of pregnancy in women who, having presented with toxoplasmosis during the 1st trimester, had been given spiramycin until delivery. Microbiological titrations of spiramycin were performed--in the maternal blood at the onset of treatment, at the time of fetal blood sampling and during delivery), in the fetal blood at the time of prenatal diagnosis of toxoplasmosis, in cord blood and placenta at birth. We were able to confirm transplacental crossing of spiramycin. Fetal blood levels during the 2nd trimester were about 47% of the maternal ones and there was no correlation between maternal and fetal levels. Levels at birth showed that (1) there was no accumulation in the course of treatment, (2) spiramycin was concentrated in placenta and (3) there was a correlation between fetal and maternal levels during the 3rd trimester.