The methods of somatic cell genetics and molecular hybridization were applied to a panel of mouse × rat hepatocyte hybrids segregating rat chromosomes to assign the rat genes coding for two serum proteins, albumin and α-fetoprotein (Alb and Afp). The molecular hybridization of DNAs from different hybrids with cloned DNA probes showed that all the hybrid clones possessing the rat Alb gene and expressing it also retained the rat Afp locus, which is not expressed in these hybrids. So the Alb and Afp genes are syntenic in the rat, as in the mouse. Furthermore, the cytogenetic analysis allowed the assignment of these two loci to rat chromosome 1