1. In cases of gas poisoning in which symptoms persist there is an increase in the number of lymphocytes, relative and absolute, in the circulating blood. In slight cases this may not be beyond the normal limits, or in excess of what may be met with from other causes. In any marked case, however, the change is sufficiently striking to be of some importance in cases where the medical officer is in doubt as to the reliance to be placed upon the statements of men complaining of having been gassed.

2. The blood change is elicited by a differential count of the leucocytes, and it may be taken that a count in which the percentage of lymphocytes approaches that of the polymorpho-nuclear leucocytes indicates that the patient is still suffering from the effects of gassing, provided always that there is no other complicating disease present which might produce a similar change. A slight relative lymphocytosis is not an uncommon finding, and particularly in men from overseas, so that no great reliance can be placed upon the sign unless it is marked, i.e. unless the percentage of lymphocytes approaches closely that of the polymorpho-nuclear cells.

3. The cell which is increased is the ordinary small lymphocyte of the blood. There may be, in some cases, a diminution in the number of polymorpho-nuclear leucocytes which will, of course, accentuate the sign, but the increase of lymphocytes is an absolute one. Moreover, it appears in cases with a high leucocyte count.

4. The change is one which develops early, probably within a month of the gassing, and continues for a long time, in cases with persistent symptoms for at least eighteen months.

5. The change appears to be independent of the kind of gas, and it is shown by patients exhibiting many varieties of symptoms.

6. It is not clear what the change is due to, but from analogy with other conditions exhibiting a lymphocytosis it is probable that chronic inflammatory change in respiratory and gastric mucous membranes is at least a factor.