The blood picture in rubella is characterised by the presence of numerous plasma and Türk cells and by the absence of toxic changes in the neutrophilic leucocytes.

Plasma cells occur also in scarlatina and in measles but with less regularity and, as a rule, in smaller numbers, but these diseases are always associated with “toxic” changes in the neutrophilic cells.

Such haematological features must be considered in relation with the clinical findings because plasma cells and toxic changes in the neutrophiles may occur in entirely different conditions, e.g. in association with the lympho cytosis following almost any acute infective malady.

The haematological method would probably find its greatest application in those cases in which the characters of the rash are not sufficiently well developed to enable a diagnosis to be made in the usual way.

Acknowledgments. My investigations were carried out at the Institute of Pathology of Charing Cross Hospital Medical School, and I am much indebted to the Director, Dr Piney, for assistance and advice, and to Dr Whittaker of the Metropolitan Asylums Board for access to clinical material. Moreover, financial assistance has been afforded by The Virol Research Scholarship which I hold.