A number of altered immunologic responses to lipopolysaccharide (LPS) in C3H/HeJ mice result from the expression in B lymphocytes of a defective genetic locus, termed Lps. Lps has been mapped to chromosome 4 between two loci, Mup-1 and Ps. As it is difficult to type individual mice for LPS responsiveness in more than one type of assay, we have utilized Mup-1 as a genetic marker to correlate LPS responses in mice to the expression of the Lps locus. Three nonlymphoid responses to LPS have been examined in 12 recombinant inbred strains of mice and in a backcross linkage analysis, and are all regulated by the expression of the Lps locus. These responses are hypothermal changes in body temperature, and the elevation in serum levels of a colony stimulating factor and the precursor of the secondary amyloid protein AA. Therefore, the initiation of LPS responses in different cell types in mice involve the expression of a common locus. These linkage studies provide a means for analyzing the genetic control of many of the diverse reactions of the endotoxic response to LPS.