The presence or absence of the anti-human T-cell leukemia virus type (HTLV-I) antibody and the HTLV-I proviral genome was examined in the offspring of inbred rabbits, which were born to HTLV-I carrier does. The results showed that not all offspring born to the carriers were infected and that not all the infected offspring seroconverted at the age of 10 weeks, which is similar to observations made in human carriers. The anti-HTLV-I antibody was assayed by indirect immunofluorescence in 55 offspring at the age of 10 weeks, which were born to B/J or (B/J x Chbb:HM)F1 seropositive HTLV-I carrier does. Twelve out of 31 offspring born from F1 x F1 mating were seropositive, whereas none of 24 offspring born from B/J x B/J mating, F1 x B/J mating, or F1 x Chbb:HM mating were seropositive. The polymerase chain reaction (PCR) method revealed the presence of the HTLV-I proviral genome in 18 out of 23 offspring born from F1 x F1 mating (F2 hybrids). In these 18 HTLV-I-infected F2 hybrids, 8 were seropositive and 10 were seronegative. The major histocompatibility complex (MHC) of these 23 F2 hybrids was analyzed by restriction fragment length polymorphism (RFLP) in southern hybridization. The results showed no close correlation of MHC with HTLV-I susceptibility or with seroconversion. Natural infection via mother-to-child transmission of virus seems to produce seronegative as well as seropositive carriers. This rabbit model may be useful for the study of seronegative virus carriers via mother-to-child transmission of HTLV-I.