Physical gene mapping by in situ hybridization is a difficult task in an all-acrocentric mouse karyotype, because all of the chromosomes are morphologically very similar. These difficulties can be overcome by using the many different metacentric Robertsonian translocation (Rb) chromosomes derived from wild mice. Here we describe the establishment of two Moloney murine leukemia virus-transformed suspension cell lines, WMP-1 and WMP-2, derived from wild mice of the strain WMP/WMP. These mice carry nine pairs of metacentric Rb chromosomes containing chromosomes 1 to 18. Chromosome 19 and the sex chromosomes are the only acrocentric chromosomes. Furthermore, a heterozygous reciprocal translocation between chromosomes 13 and 17 involved in two Rb chromosomes is present in this stock and provides additional marker chromosomes. The chromosome designation of these mice is Rb(10.17)9Mpl Rb(13.15)10Mpl T(13.17)1Lub.