We applied the polymerase chain reaction (PCR) method to detect gag, env and pX sequences of human T cell leukemia virus type I (HTLV-I) provirus in peripheral blood lymphocytes of seronegative infants born to HTLV-I seropositive mothers. Out of 22, five subjects were found to contain the HTLV-I provirus genome. Two of the five cases were judged to be negative for not only anti-HTLV-I antibodies but also the viral antigens on cultivated lymphocytes by the conventional antibody/antigen detection methods. These results indicate that PCR is of great use as a simple and highly sensitive method detect HTLV-I infection.