For over 40 years germ-cell mutagenesis experiments have generated many new mutations at the brown (b or Tyrp1) locus on mouse Chromosome (Chr) 4. These mutations, many of which are deletions, were recovered by the specific-locus mutagenesis technique. Previous analysis of a panel of brown deletions, generated at Oak Ridge, has enabled both a preliminary molecular and a functional map around the locus to be generated. We have used a panel of hybrid DNA from 25 Oak Ridge deletions, where the deleted chromosome was heterozygous with a Mus spretus chromosome, to map polymorphic markers including microclones, microsatellites, and cloned DNA markers. We have generated a fine structure map, based on 25 new markers, of an 8.5-cM region surrounding the brown locus. This map will prove useful in future mapping studies of this region and in the isolation of the genes that lie within it.