The blood picture of the echidna Tacchyglossus aculeatus, one of the two monotremes still extant, was studied on forty-two apparently healthy and diseased specimens.

In spite of the reptilian and avian attributes, the red and white cell characteristies and counts, response to infection, etc. of this prototherian correspond typically to thoso of higher mammals.

Compared with Eutheria the average “normal” haemoglobin content was high (17.4 gram per cent). Nucleated red cells were only observed in marked anaemia and reticulocytes were almost never seen. The prevailing polymorph was a mature neutrophil. Eosinophils were uncommon and basophils rare and then only in disease.

A varying degree of piroplasmosis was present in most of the animals examined. This did not have a definite influence on the blood picture if light or moderate. The haemoglobin content diminished considerably if the echidna was kept in captivity for more than two months.