In a lecture delivered recently in Cambridge, I had occasion to demonstrate the degenerative changes undergone by P. canis and Tr. brucei in the blood of animals which had been subjected to curative treatment with drugs. These changes have not as yet been depicted. They are, in my opinion, of more than passing interest, since they may help us to distinguish normal from abnormal parasites in untreated animals. Without doubt there occurs a certain death-rate amongst blood parasites under natural conditions; what this death-rate is we do not know, but appearances which cannot be regarded otherwise than as degenerative are not infrequently encountered. On the other hand, there is always a danger that false interpretations may be placed upon abnormal or degenerative forms, and of this I fear there is ample evidence in current literature. In publishing this note I merely desire to draw attention to what may prove to be a useful method of differentiating some of the normal from the abnormal appearances presented by Haematozoa.