Between July, 1994 and March, 1995, 23 heart blood samples from fresh abortuses of HIV-1 seropositive pregnant women after elective termination of pregnancy between 18 and 25 weeks of gestation by prostaglandin E1 analogue vaginal administration were examined for polymerase chain reaction (PCR) of HIV-1 genome and p24 antigen to investigate the transplacental transfer of HIV-1 infection. All samples of fetal heart blood were positive for HIV-1 antibody (ELISA), but negative for PCR and HIV-1 p24 antigen assay. These negative results could be due to the lack of the virus in the peripheral blood or to a viral load low enough to be undetectable by PCR method at mid-trimester gestation and suggest that HIV-1 vertical transmission occurs mostly during the last trimester of pregnancy and/or at delivery.