Abstract C3H/HeJ mice are poorly responsive to all of the biologic effects of bacterial lipopolysaccharides (LPS) and are also, paradoxically, highly susceptible to Salmonella typhimurium infection. The defective response of these mice to LPS is governed by the Lpsd allele, but the gene(s) controlling their response to the bacterium has not been identified. Therefore, genetic analyses were performed to characterize the gene(s) responsible for the susceptibility of C3H/HeJ mice to S. typhimurium and to assess the influence of the Lpsd allele on this sensitivity to infection. The 50% lethal dose (LD50) of S. typhimurium for C3H/HeJ mice was <2, whereas the LD50 for other genetically related but LPS-responsive C3H substrains was ≥2 × 103. The susceptibility gene was distinct from both of the genes previously reported to confer susceptibility to murine typhoid (Ity* and xid). The F1 progeny derived from crosses of C3H/HeJ mice with the highly susceptible C57BL/6J strain (Ity8/Ity8) were resistant (LD50 ≥8 × 103), which indicated that the C3H/ HeJ susceptibility gene was distinct from Ity8. Furthermore, the gene(s) that governed the susceptibility of C3H/HeJ mice to S. typhimurium was also distinct from the X-linked susceptibility gene (xid) because both (C3H/ HeJ × C3H/HeN)F1 males and females were resistant (LD50 ≥ 1 × 104). To determine whether susceptibility to S. typhimurium was due to the expression of Lpsd, a backcross linkage analysis was performed by using the progeny derived from C3H/HeJ and (C3H/HeJ × C57BL/6J)F1 parents. There was a close correlation between a defective LPS proliferative response and susceptibility, since 13 out of 14 of such mice succumbed to infection whereas only 2 out of 13 of LPS-responsive animals died. The susceptibility of backcross mice was also closely linked to another chromosome-4 locus, Mup-1a. These data strongly suggest that the susceptibility of C3H/HeJ mice to S. typhimurium is due either to the Lpsd allele or to a closely linked gene.