1. Mepacrine has a damaging action upon both vegetative forms and cysts of Giardia intestinalis. The action upon the former is rapid; but the action upon the cysts is gradual, and it takes 4 days of incubation in a 0·06% mepacrine suspension before all normal cysts disappear. The degeneration of cysts is accelerated by raising the concentration of mepacrine to which they are exposed.

2. Mepacrine produces a characteristic degeneration of the cyst which can be seen both in vitro and in vivo. This appearance develops gradually. In affected cysts the wall appears unaltered; but the protoplasm becomes shrunken and yellow, occupying only a small part of the internal cyst space. Whether shrunken or not, protoplasm stained by mepacrine is probably too much injured to survive, although the intense yellow colour makes it difficult to demonstrate eosin staining.

3. When parasites showing the characteristic abnormality are seen in the faeces they should be recognized as degenerate cysts, and further specimens should be examined before the patient is pronounced free from infection.

4. The usual method of treating a Giardia infection with mepacrine in a dosage of 0·05–0·3 gm. daily according to age, given by mouth for 6 days successfully eradicates the parasite, which is killed in both cystic and vegetative stages, but this dose is near to the minimum effective and neither time nor dosage can safely be reduced if a reliable cure is wanted.

5. Caustic soda destroys the whole cyst fairly rapidly, while a detergent had a damaging action on the protoplasm. Alkaline detergent solutions may therefore be recommended for the cleansing of infected utensils and premises. Dettol and Lysol kill cysts within 30 min. (judged by eosin staining), and 1% solutions of these disinfectants appear as effective as 5%. Utensils should be left to soak in the disinfecting solution since some cysts are slower to be affected than are others.

6. Cysts of Giardia are fairly resistant to acid, and up to 50% of them survived 4 hr. incubation in hydrochloric acid at a concentration of 0·26 gm.%, which corresponds to the upper limit of normal human gastric acidity (Harrison, 1947).

7. The cysts were damaged in different ways by different substances. In some cases the cyst was unchanged in appearance and yet rendered susceptible to eosin staining, in others the nuclei alone stained with eosin. Sometimes the protoplasm shrank within an intact cyst wall, and sometimes the wall was attacked directly.

8. In each sample of faeces studied in these experiments the Giardia cysts present appeared to be of unequal vitality, since, whatever the test substance to which they were exposed, and whatever form of degeneration developed, some cysts were affected hours or days before others showed any changes. It is conjectured that the age of the parasite since encystment may influence its resistance to damage.

Grateful acknowledgements are made to the practitioners and Medical Officers who co-operated in the following-up of cases of Giardiasis after they had been treated by mepacrine.