In 1922 Iwaschenzoff described from Russia an epidemic of relapsing fever attended by high mortality. From many of the sufferers organisms of the Salmonella group were isolated. These organisms, termed S. paratyphus N1, were apparently responsible for the severity of the cases. In 1921–2 the mortality rate in uncomplicated cases of relapsing fever was 5·5 per cent.; whereas it was 50 per cent, in cases showing secondary infection by Paratyphus N bacilli.

In 1927 we undertook certain work at the request of the Medical Research Council, as part of an international investigation into the standardisation of scarlet fever reagents promoted by the Health Organisation of the League of Nations. The laboratory work was carried out by the late Dr G. R. James, and his untimely death caused the suspension of the research. A great many observations, however, had been made. Since many of these have a general interest, and since up to the time of writing no completely satisfactory experimental method of assay has been devised, we have no hesitation in adding to the volume of evidence that is accumulating with regard to the methods at present in use.