The present study examined the influence of the mouse Lps genotype on the interleukin-6 (IL-6) and polymorphonuclear leucocyte (PMN) responses to mucosal Escherichia coli infection. Lipopolysaccharide (LPS) responder C57BL/6J (Lpsn, Lpsn) and LPS non-responder C57BL/10ScCr (Lpsd, Lpsd) mice were inoculated intravesically with Escherichia coli Hu734. The secretion of IL-6, the recruitment of PMNs into urine, and the bacterial clearance from the kidneys and bladders were compared between the two mouse strains at 2, 6 and 24 h after infection. The C57BL/6J mice showed a high PMN response and rapid clearance of bacteria from the kidneys and bladders. In the C57BL/10ScCr mice the PMN response was low and infection remained. This supported a role of the Lps genotype in these events. The IL-6 levels remained low after infection in both LPS responder and non-responder mice, but became elevated in the animals which were accidentally traumatized during infection. The IL-6 response to trauma alone was independent of Lps genotype, but the response to trauma and infection combined differed between the mouse strains. The IL-6 response to trauma and infection was more rapid in the C57BL/6J than in the C57B1/10ScCr mice. The traumatized and infected animals did not clear the infection as efficiently as the non-injured animals in both backgrounds. The difference in PMN recruitment and susceptibility of infection between LPS responder and non-responder mice in the C57 Black background followed the pattern previously seen in the C3H mouse background and suggested that these events were under Lps gene control. An effect of the Lps locus on the IL-6 response could be detected only in traumatized and infected animals.