SUMMARY: A loss or reduction in the O side-chain material of the cell-wall lipo-polysaccharide is known to abolish or reduce the virulence of Salmonella strains. The present report concerns the effect on virulence of altering the quality of smooth-type O side-chains in a basically virulent line. The original locus determining the structure of -specific repeating units was replaced either in transduction or in conjugation by the wild-type locus of group D (O antigens ), or of group C (O antigens ). The LD 50 values of the recombinants or transductants were about 10 and like those of the parent, whereas the LD 50 values of the transductants were about 10, and the LD 50 values of the recombinants and transductants were over 10. The reduced virulence of both and recombinants could be restored to the original level by reintroducing the locus into these strains through conjugation. It seems, therefore, that different kinds of O side-chains confer different degrees of virulence on


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