It is known that mice are not permissive to infection with Human T-cell Leukemia Virus I (HTLV-I). In this work, BALB/C mice were immunized at least three times with lethally irradiated HTLV-I-transformed rabbit cell lines RH/K30 or RH/K34, incorporated with or without Freund`s adjuvant. Anti-HTLV-I antibodies were found in all the sera of immunized mice. Two mice were used to produce monoclonal antibodies at two different times. One was immunized with RH/K30 and the other with RH/K34 cell lines. The spleens of these two mice were seven to ten times larger than the normal mouse spleen. Many cell hybridomas presented an elevated number of climbing cells. Two of them, L-158 and R-164, were chosen to be studied in this report. Both cell lines produced the HTLV-I p19 antigen in their supernatant for prolonged periods of time, for instance, L-158 produced the antigen for more than one year. Flow cytometric analysis results indicated that these cell lines were clearly positive for H2 class I molecules of BALB/C mouse. Electron microscopic analysis showed the presence of virus particles associated with the infected cells. Apoptotic cells were also observed. The results indicate that in these conditions, normal adult BALB/C mice could be infected by HTLV-I.