Quantitative experiments on the interaction of Salmonella choleraesuis and Salmonella dublin with porcine and bovine intestinal epithelia yielded no evidence to suggest that host restriction of S. choleraesuis and S. dublin for pigs and calves respectively could be explained in terms of the patterns of intestinal invasion observed in ligated ileal loops in vivo, at 3 h after challenge. No evidence was found to support the idea that Peyer's patches, or specifically M cells, are the major route of entry for these serotypes in vivo. Three hours after loop inoculation, each serotype was recovered in comparable numbers from either absorptive or Peyer's patch mucosae present in the same ileal loop, indicating that both types of tissue are involved in the early stages of the enteropathogenic process induced by both serotypes. More detailed transmission electron microscopic (TEM) analyses of follicle-associated epithelia (FAE) challenged with S. choleraesuis showed that in the same region of FAE, organisms invaded both M cells and enterocytes directly; comparable detailed TEM studies with S. dublin could not be carried out because of the tissue-destructive properties of this serotype. S. dublin was clearly more histotoxic than S. choleraesuis as had previously been found in rabbits: this difference is almost certainly due to a tissue-damaging toxin which is neither host nor gut-tissue specific. The tissue-destructive potential of S. dublin has profound implications for the measurement of and the assignment of significance to the invasiveness of S. dublin. S. dublin was nearly always seen entering gut cells in micro-colonies whereas S. choleraesuis entered mainly as single organisms or small groups of two or three.