Perinatal infection of human T-lymphotropic virus type I, the etiologic virus of adult T-cell leukemia/lymphoma. DNA amplification of specific human T-lymphotropic virus type I sequences. A gene amplification technique, polymerase chain reaction, was used to detect human T-lymphotropic virus type I (HTLV-I), the etiologic agent of adult T-cell leukemia/lymphoma, in mononuclear cells in peripheral blood and breast milk of ten HTLV-I carrier gravida. The DNA in umbilical cord blood mononuclear cells of the neonates born to the HTLV-I carrier gravida were also amplified and examined for the possibility of HTLV-I infection via placenta during pregnancy. The HTLV-I sequences were detected both in the peripheral blood and milk of all ten carrier gravida by Southern blot analysis of amplified DNA. However, HTLV-I proviral DNA could not be detected in the cord blood of the carriers' neonates, indicating that transplacental infection of HTLV-I should be rare and that postpartum infection via breast milk is a likely major perinatal transmission route. 