The rfb gene cluster and the rfc gene of Salmonella enterica were introduced earlier into an invasive Shigella dysenteriae 1 strain by triparental cross. Antiserum was raised in rabbit against lipopolysaccharide isolated from the hybrid strain. Both the hybrid and the invasive S. dysenteriae 1 strain were found to have a titer of 1:2560 while for S. enterica, it was 1:640. Ligated ileal loops were prepared in rabbit, which were inoculated with 10(8) CFU ml(-1) each of the hybrid strain, and invasive S. dysenteriae 1 strain used as positive control. Escherichia coli K12 was also used as a negative control. After 18 h, the fluid accumulation ratios were 0.2 and 1.6 for hybrid and invasive strains of S. dysenteriae 1, respectively. Rabbit intestinal mucosa infected with hybrid S. dysenteriae 1 strain showed the presence of intact villus tips and unruptured intestinal mucosa whereas total necrosis of intestinal mucosa and villi was observed in the S. dysenteriae 1-infected region.