Abstract
1.Phase variation of the somatic antigen I inBact. paratyphosumA and of the component XII2inBact. typhosumand British strains ofBact. enteritidishas been confirmed.2. Strains ofBact. enteritidisisolated in India by blood culture from cases of invasive disease in man fall into two serological types, neither of which undergoes demonstrable somatic phase variation.3. The probable mechanism of this type of variation is discussed in the light of experimental findings.4. The effect which phase variation may have on the clarity of end-points in agglutinin titrations and on the serological standardization of Salmonella O-suspensions is demonstrated.5. The variation is briefly discussed from the points of view of Salmonella philogeny and of human Salmonella infection.I am indebted to the D.M.S. in India for permission to publish this paper.