An irradiation-reduced somatic cell hybrid mapping panel was constructed of BALB/c mouse Chromosome 1. Nineteen hybrids were selected from a pool of 292 clones to generate a fine structure physical map of the distal 40 cM of the chromosome. The hybrids contain mouse DNA fragments only from Chromosome 1, ranging from approximately 5 cM to approximately 20 cM. Utilizing a viral infectibility assay, a cellular receptor gene, Rmc-1, for the MCF class of murine retroviruses was found to be linked to Lamb2, in the region between the Lamb2 and Bxv-1 loci. In addition, analysis of the hybrid mapping panel resulted in the remapping of three loci, Atpb, Ly-5, and Pmv-24, as compared to the mouse linkage map. Two previously unmapped endogenous proviruses are also putatively assigned positions on the chromosome.