The polymerase chain reaction (PCR) assay was used to investigate the possibility of HIV1 DNA detection in uncultured peripheral blood mononuclear cells from newborn infants and children of HIV-infected mothers. HIV1 DNA sequences were detected in mononuclear cells of six of fourteen symptom-free newborn infants of seropositive mothers. Only one of these infants had detectable HIV antigenaemia. In addition, HIV1 DNA was identified in the mononuclear cells of five of ten children (2-5 years old) of infected mothers who had become seronegative 12-15 months after birth; among these, four children had only mild clinical features related to HIV infection, while the other had none. HIV1 DNA was shown in all of eight seropositive children with HIV infection and none of fifteen normal seronegative controls. The PCR assay thus provides an early and direct identification of HIV infection in newborn infants and seronegative children born to infected mothers.