A major obstacle to understanding AIDS is the lack of a suitable small animal model for studying HIV-1 infection and the subsequent development of AIDS, and for testing diagnostic, therapeutic, and preventive modalities. Our goal is to produce a rabbit model for the study of AIDS. Here we report on the generation of transgenic rabbits that express the human CD4 (hCD4) gene. The transgene, which contains the coding region for hCD4 and approximately 23 kb of sequence upstream of the translation start site, was used previously to direct hcD4 expression on the surface of CD4+ T cells of transgenic mice (Gillespie et al., 1993: Mol Cell Biol 13:2952-2958). The hCD4 transgene was detected in five males and two females derived from the microinjection in five males and two females derived from the microinjection of 271 rabbit embryos. Both hCD4 RNA and protein were expressed in peripheral blood lymphocytes (PBLs) from all five males but neither of the females. Human CD4 was expressed on PBLs from F1 offspring of all founder males. T-cell subset analysis revealed that hCD4 expression was restricted to rabbit CD4 (rCD4) expressing lymphocytes; mature rCD4- rCD8+ lymphocytes did not express hCD4. In preliminary studies, PBLs from hCD4 transgenic rabbits produced greater amounts of HIV-1 p24 core protein following HIV-1 infection in vitro than HIV-1 p24 antigen in nontransgenic rabbit infected cultures. These results extend to rabbits our previous observation that this transgene contains the sequence elements required for high-level expression in the appropriate cells of transgenic mice. Furthermore, these and previous studies demonstrating that expression of hCD4 protein enhances HIV-1 infection of rabbit T cells in vitro, coupled with reports that normal, nontransgenic rabbits are susceptible to HIV-1 infection, suggests that the hCD4 transgenic rabbits described herein will have an increased susceptibility to HIV-1 infection. In vivo HIV-1 infection studies with these rabbits are under way.