An experimental rat model for vertical transmission of human T cell leukemia virus type I (HTLV-I) was used to examine whether the infection of offspring derived from HTLV-I carrier rats could be established during the suckling period. MT-2 (2 × 107) cells were injected into 5-week-old rats twice, at 2-week intervals. HTLV-I infected or non-infected female rats were mated with HTLV-I carrier male rats. The titer of serum antibodies against HTLV-I in the offspring derived from non-infected dams was less than 1:16 by the agglutination test during the suckling period. The serum antibodies of the offspring derived from the infected dams was less than 1:32 at 1 day after birth and increased steadily to 1:2048 at 14 days. However, the HTLV-I proviral sequences were not detected in any organs of the offspring during the suckling period as determined by the nested double polymerase chain reaction (PCR). These findings indicate that maternal antibodies against HTLV-I were not readily transmitted through the placenta and that the anti-HTLV-I antibodies in the offspring came from the milk of the dams. Furthermore, the HTLV-I infection in the offspring that was derived from the carrier dam may not be established during the suckling period but after weaning.