The cultural, serological and immunological properties of eight rough mutants of S. gallinarum
were compared with the parent smooth strains as well as with a reference rough mutant (S. gallinarum
strain 9R). The rough mutants could be readily differentiated from the smooth ones by means of their ability to agglutinate in acriflavine, but they could not be distinguished from each other although they were obviously not identical.
One of these mutants was selected as a standard vaccine strain on the basis of its excellent immunizing properties and low virulence but some of the other mutants would also have been satisfactory.
The nature of the immunity induced by these mutants is discussed with particular reference to the possible role of cellular immunity and cytophilic antibody.