The inv gene encodes the protein invasin, which is the primary invasion factor for Yersinia enterocolitica in vitro and in vivo. Previous studies of Yersinia species have shown that inv expression and entry into mammalian cells are temperature regulated. Invasin production is reduced at the host temperature of 37 degrees C as compared to production at ambient temperature; consequently, this study was initiated to determine whether other host environmental signals might induce inv expression at 37 degrees C. An inv::phoA translational fusion was recombined on to the Y. enterocolitica chromosome by allelic exchange to monitor inv expression. Molecular characterization of expression of the wild-type inv gene and the inv::phoA fusion showed that invasin is not produced until early stationary phase in bacteria grown at 23 degrees C. Y. enterocolitica grown at 37 degrees C and pH 5.5 showed levels of inv expression comparable to those observed in bacteria grown at 23 degrees C. An increase in Na+ ions caused a slight increase in expression at 37 degrees C. However, expression at 37 degrees C was unaffected by anaerobiosis, growth medium, calcium levels, or iron levels. Additionally, Y. enterocolitica expressed invasin in Peyer's patches two days after being introduced intragastrically into BALB/c mice. These results suggest that invasin expression in Y. enterocolitica may remain elevated early during interaction with the intestinal epithelium, a site at which invasin was shown to be necessary.