The relative levels of invasiveness of two bovine isolates each of Salmonella typhimurium and Salmonella dublin and of invH mutants of S. typhimurium were determined in MDCK and Int 407 cultured-cell assays and in bovine ileal loops. S. dublin was found to be significantly less invasive in cultured cells than S. typhimurium, but this difference was not observed in bovine intestines. The invH mutants exhibited a significant reduction in invasion in both cultured cells and bovine intestines. The invasive phenotypes of the strains were confirmed by fluorescent microscopy and scanning and transmission electron microscopy. The wild-type strains were observed in the laminae propriae of the intestinal villi, while in contrast the invH mutants were generally associated with the enterocyte layer. The degree of damage in the bovine ileum was related to the magnitude of the invasion. There was no difference in the amount of S. typhimurium or S. dublin recovered from the bovine ileum either with or without Peyer's patches 3 h after inoculation of the loop.