The expression of a defective LPS response gene Lps and the major urinary protein (Mup-1) are concordantly inherited in backcross (C3H/HeJ × C57BL/6J)F1 × C3H/HeJ mice, indicating genetic linkage of these loci. Mup-1 is known to be linked to the brown coat color locus on chromosome 4 in mice; thus Lps can now be assigned to chromosome 4. A value of 0.06 ± 0.02 has been estimated for the recombination frequency between Mup-1 and Lps. We have used the polysyndactyly (Ps) mutation further to localize Lps on chromosome 4. Lps is located between the Mup-1 and Ps loci.