The karyotype of HY 5/3--4 hybrid cells formed by fusion of diploid and heteroploid murine cells (LS/BLxR-AG/20) consisted of 60--75 chromosomes (modal number 72). The G-banding technique made it possible to identify the origin of the chromosomes; it is concluded that due to chromosomal segregation the hybrid genome lost preferentially the chromosomes derived from the diploid parental line LS/BL. Even after long-term culture in vitro, the chromosomal complement of hybrid cells retained all bi-armed chromosomes and most of the telocentrics, derived from the heteroploid parental cells.