We have found the in vivo-regulated nirB promoter (P_nirB) to be effective for directing expression of a number of antigens in salmonella in vivo. We wished to determine if other in vivo-regulated promoters have utility for antigen expression in salmonella and to compare the effectiveness of these promoters with that of P_nirB. To this end, we have devised a scheme that allows the promoter element of the P_nirB-fragment C plasmid pTETnir15 to be swapped with other promoters of interest. We demonstrate the usefulness of this system by replacing P_nirB with P_htrA to create plasmid pTEThtrA1.htrA is a stress response gene that is required for virulence of salmonella in mice and survival within macrophages. Expression of fragment C in Salmonella typhimurium BRD509 (aroA aroD) harboring pTEThtrA1 (strain BRD937) correlated with growth temperature in vitro. A comparison was made of the immune responses to fragment C elicited in mice immunized orally with BRD937 or BRD847 (BRD509/pTETnir15) or subcutaneously with purified fragment C plus alhydrogel. High levels of anti-fragment C antibodies that persisted for at least 12 weeks were present in all groups of mice. Vaccination with BRD937 was the most effective means of immunization: the serum immunoglobulin G (IgG), IgA, and IgM anti-fragment C titers were higher in the BRD937-immunized mice throughout the duration of the study than in mice in the other groups. The kinetics of the serum anti-fragment C responses were different in different groups. The response was most rapid in the BRD937 group, with the titers almost at peak levels at 2 weeks postimmunization. Only the mice immunized with BRD937 or BRD847 developed an intestinal IgA response to fragment C. Again, the response was superior in the BRD937 group. The peak of the intestinal response was delayed with respect to the serum response. Analysis of the IgG subtype response to fragment C revealed a dominant IgG2a response in the salmonella-immunized mice, indicating a type 1 helper T-cell response to fragment C, whereas the major subtype in the group parenterally immunized with fragment C plus alhydrogel was IgG1. The IgG1/IgG2a ratio was much higher in sera of BRD937-immunized mice than in sera of BRD847-immunized mice. At 15 to 20 weeks after immunization, the mice immunized with BRD937 or BRD847 were solidly immune to tetanus toxin and salmonella. The immune responses to fragment C seen in mice immunized with BRD937 are the strongest we have observed and indicate that the htrApromoter may be very useful for expressing foreign antigens in salmonella vaccine strains.