By constructing Salmonella strains with a FLAG epitope sequence inserted in frame into the SPI-1 genes prgI, sipA, sipB, sopE2, spaO, and sptP, and characterizing the expression of the tagged proteins in vitro and in vivo, we provide direct evidence that PrgI and SipB are expressed in vivo in both the early and late stages of bacterial infection. This evidence concerns the gene SPI1 and bacterial infectious disease.