Hoechst 33258 fluorescent staining can be coupled with G-banding to identify the chromosomal contribution of each parent in mouse-rabbit hybridomas. A fast and essentially complete segregation of rabbit chromosomes is obtained in these cells. The rabbit X chromosome is preferentially maintained in media imposing HGPRT activity for cell growth. Mouse-rabbit hybridomas, some of which secrete rabbit immunoglobulin chains, should be a convenient material for the identification of chromosomes governing rabbit Ig chain synthesis.