a1 From the Central Enteric Reference Laboratory and Bureau, Public Health Laboratory Service (Medical Research Council), London
1. Type F1 will appear in old Dorset egg cultures of Type F2 after storage for months or years at room temperature in the dark.
2. It is probable that a similar change is responsible for the appearance of Types F1 and F2 in the same carrier and outbreak of typhoid fever.
3. The two types are stable when growing logarithmically in the pure state.
4. There is also evidence that the two types are stable under ordinary field conditions.
5. Logarithmic growth of F1 in the presence of F2 results in the disappearance of F1.
6. This change is caused by a phago carried by F2, which converts F1 into F2.
7. The significance of these observations is discussed. It is suggested that the two types retain their status in the typing scheme.
(Received June 09 1951)